APPARATUS AND METHOD FOR ELECTRIC HOT WATER HEATER PRIMARY FREQUENCY CONTROL

Abstract

An energy storing apparatus is provided which comprises: a first region having a thermostat which includes: a temperature sensor; a controller coupled to the temperature sensor; and a power-converter coupled to the controller; and a second region thermally coupled to the first region. An apparatus is provided which comprises: an interface to be coupled to an electric grid; a temperature sensor; a controller coupled to the temperature sensor; and a power-converter coupled to the controller, wherein the power-converter is to couple via passive devices to a heating element, wherein the power-converter is to apply power to the heating element according to a temperature sensed by the temperature sensor and a frequency of the electric grid.

Description

CLAIM OF PRIORITY

This application claims benefit of priority of U.S. Provisional Application No. 62/239,765 filed 9 Oct. 2015, which is incorporated by reference in its entirety.

GOVERNMENT LICENSE RIGHTS

This invention was made with support from the United States Government under Grant No. 51205A (ECCS-0846533) awarded by the National Science Foundation (NSF). The Government has certain rights in the invention.

BACKGROUND

One limitation to large-scale integration of wind, solar, and marine energy is their inherent variability. Presently, variability of the load or generation on the electrical grid is managed through fast acting (i.e., spinning) reserve generation. For large-scale integration of variable renewable power sources, the variability can exceed system limits. Energy storage is one means of managing the additional variability, but at present large dedicated energy storage systems are significantly more expensive than generation. A much more cost effective solution is to utilize demand response (i.e., controlling the electrical load to increase or decrease in response to the instantaneous availability of renewable generation). One demand response resource that stands out in its scope, flexibility, and simplicity is an electric water heater.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments of the disclosure will be understood more fully from the detailed description given below and from the accompanying drawings of various embodiments of the disclosure, which, however, should not be taken to limit the disclosure to the specific embodiments, but are for explanation and understanding only.

FIG. 1 illustrates a plot showing a transition drop in frequency for a Western Grid Interconnect (WECC) model.

FIG. 2 illustrates a reduced-order model of WECC which includes system inertia, load frequency response, and primary response of three classes of generators: slow, medium, and fast, according to some embodiments.

FIG. 3 illustrates a plot showing a comparison of the base case frequency response and the reduced-order WECC model, according to some embodiments.

FIG. 4 illustrates a plot showing response of load for the three classes of generators to a simulated loss of generation.

FIG. 5 illustrates a water heater with upper and lower heating elements, according to some embodiments.

FIG. 6 illustrates a plot showing available aggregate water heater power capacity as a function of aggregate water heater State of Charge (SOC).

FIG. 7 illustrates a simplified WECC model with aggregate water heater model, in accordance with some embodiments.

FIG. 8 illustrates a plot showing a frequency response of the WECC model to a Palo Verde outage under three difference scenarios.

FIG. 9 illustrates a plot showing water heater power in response to change in frequency due to loss of generation for two cases.

FIG. 10A illustrates a water heater with power electronics and control, according to some embodiments of the disclosure.

FIG. 10B illustrates an apparatus for the lower thermostat, according to some embodiments of the disclosure.

FIGS. 11A-B illustrate a plot showing power and frequency simulation results of 1000 water heaters over a 24-hour period.

FIG. 12 illustrates a water heater and operation of its thermostats, according to some embodiments of the disclosure.

FIG. 13 illustrates a circuit level architecture of the power electronics and control for the lower thermostat, according to some embodiments of the disclosure.

FIG. 14 illustrates an architecture of the lower thermostat, according to some embodiments of the disclosure.

FIG. 15A-B illustrate flowcharts of a method operation of the water heater, according to some embodiments of the disclosure.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

A powerful and flexible feature of residential hot water heaters is the ability to heat both the upper part of the water tank—where the hot water is withdrawn—and the lower part of the tank—where fresh cold water is drawn in—separately with upper and lower heating elements. Some embodiments take advantage of this feature to provide a significant amount of energy storage in the lower part of the water tank, while maintaining the desired hot water temperature for the resident.

There are an estimated several million electric hot waters in the Pacific Northwest of the United States of America. Each residential hot water heater is typically rated, for example at 4.5 kW (kilo-Watt). In this example, if a 15 degree Celsius to 50 degrees Celsius temperature range is allowed in the bottom two-thirds of a typical residential water heater tank, each water heater represents over 5 kWh (kilo-Watt hour) of energy storage. The total water heater potential for the Pacific Northwest is then in the range of 10 GW (Giga-Watt) of power capacity and 10 GWh (Giga-Watt hour) of energy storage.

In some embodiments, the energy storage potential of hot water heaters is realized by manipulating the temperature of the water in the tank. An example method is to heat the water to a high temperature when there is a large amount of renewable power available on the electric grid. This increases the electric grid load to match the available renewable power generation. At a later time, when there is less renewable power generation available (e.g., the wind speed decreases and wind power decreases, or when sun sets and solar power decreases), the water temperature is allowed to coast down to a lower temperature, thus allowing more water heaters to be off during this time, thus allowing the electric grid load to better match the reduced renewable generation.

Stable operation of the electrical grid requires that the generated electrical power is equal to the load power. Deviations in this balance (in which the generation of electrical power may be greater than the load, or the load is greater than the generated electrical power) cause the rotating electrical machinery to increase or decrease in speed, which in turn increases or decreases the electric grid frequency. Large deviations in the electric grid frequency can result in failure of the electric grid connected equipment, or collapse of the entire electric grid. Uncertainty in the generation of electric power or load imbalance, which can be caused by sudden loss of generation of electric power (e.g., from a failure of a generator), loss of transmission (e.g., like a tree failing on a power line), or changing wind conditions (e.g., when wind speed changes causing wind power unit to stop generating power), can cause the electric grid frequency to deviate.

Special generation called “reserve” generation is kept alone to accommodate for these changing conditions. Reserve generation tends to be more expensive and less efficient than base load generation. Electric hot waters can provide some reserve generation functionality at a small fraction of the cost of the building new electric generation facilities, in accordance with some embodiments.

The overall aggregate electrical load connected to the electric grid has a frequency characteristic. The aggregate electrical load increases slightly with an increase in frequency, and decreases slightly with a decrease in frequency. This is largely due to the electrical motor component of the aggregate load, as these loads spin slightly faster with a higher grid frequency, and vice versa.

In some embodiments, an apparatus is provided that generates a frequency dependence in electric hot water heaters, which as non-rotating load do not have any frequency characteristics. As such, the apparatus allows the electric hot water heaters to participate in frequency regulation of the electric grid. The circuit(s) of various embodiments are responsive to both temperature and electric grid frequency, lending some stability and inertia to the electric grid.

The apparatus of some embodiments retrofits existing water heaters, and because the apparatus operates on the electric grid frequency, no additional communication signals (in or out) are needed. As such, in some embodiments, electric hot water heaters participate in frequency regulation without the Internet or dedicated communication infrastructure.

Some embodiments provide mechanisms to allow the temperature of the bottom portion of the water heater tank to have a greater variation than is currently allowed. In some embodiments, if the variation in temperature in the lower portion of the tank is kept within the appropriate bounds, it has little impact on the temperature in the top portion of the tank where water is withdrawn for the consumer. This is due to stratification of water temperature layers in the tank, and the fact that the upper heating element has priority.

In some embodiments, the lower thermostat of an existing off-the-shelf residential hot water heater is replaced with a low-cost embedded controller and power converter. In some embodiments, the low-cost embedded controller and power converter includes a temperature sensor. In some embodiments, the low-cost embedded controller and power converter is operable to support the operation of the lower part of the tank such that the electrical load of the water heater depends on the frequency of the electrical grid. In some embodiments, the low-cost embedded controller and power converter is a replacement of the standard thermostat in the lower part of the tank.

In some embodiments, the controller detects temperature of the lower tank, and also the frequency of the electrical grid (which is available on the relayed power connection from the upper thermostat, for example). In some embodiments, when the electric grid frequency is above 60 Hz (Hertz), the power converter increases the power to the lower element. In some embodiments, when the electric grid frequency is below 60 Hz, the power converter decreases the power to the lower element.

There are many technical effects of the methods and apparatus/circuit(s) of the various embodiments. In some embodiments, the apparatus retrofits existing thermostat technology, effectively leveraging massive energy storage potential at a very low cost (e.g., effectively 5 kWh of energy storage capability is added for a cost of tens of dollars). In some embodiments, the apparatus can be easily installed in typical electric hot water heaters. In some embodiments, the apparatus allows the water heater energy storage to participate in the stability of the electric grid using merely the detection of electric grid frequency. In some embodiments, no additional communication overhead and control is necessary (e.g., no Internet connections, no dedicated communication networks and protocols are needed). The apparatus of the various embodiments may not modify the proven and fundamental base control scheme for hot water heaters. For example, the top or upper thermostat still operates as before, and still has priority in maintaining desired water temperature for the consumer.

Various embodiments add frequency response to a large portion of the electrical load that currently does not exhibit dependence on frequency of the electrical grid. Load frequency response stabilizes the grid. In some embodiments, the apparatus can be easily augmented to emulate inertia. Inertia stabilizes the grid.

In some cases, if balancing area operators allow some variation of frequency as a natural signal of renewable generation and load balance, the advanced hot water heaters of the various embodiments may automatically operate as energy storage devices to help integrate more renewable power.

Here, the calibration of a reduced-order model of the Western Grid Interconnect (WECC), and the usage of that model to investigate the impact of large-scale integration of domestic hot water heaters for energy storage and frequency response are described. The simulations suggest that even a very modest penetration of water heaters (approx. 500 MW (Mega-Watt) power capacity and 555 MWh (Mega-Watt Hour) energy capacity), can have a significant impact on reducing the frequency deviation nadir and the settling frequency deviation in response to a large power generation outage. With hot water heaters responding at the standard of generation rated capacity per 3 Hz deviation, there is a small but measurable reduction in the 60 Hz deviation nadir, from 59.666 Hz to 59.673 Hz, for example.

If the water heaters are set to respond much more aggressively, e.g., full rated capacity per 0.33 Hz deviation, the water heaters can contribute their full 250 MW capacity (assuming 50% SOC—state of charge) and the frequency nadir improves from 59.666 Hz to 59.697 Hz, which is a 10% improvement in the deviation from 60 Hz. Domestic hot heaters represent a very large, fast, and flexible resource that should be able to be utilized with a much lower implementation cost than new energy storage.

Domestic water heaters represent a huge potential form of energy storage, both in terms of energy capacity and power capacity. In this disclosure, in some embodiments, merely the time scale that covers primary response is considered. Generally, electric grid reserve generation can be classified into four timescales described here.

First, inertial response is a timescale which is not dispatched, but instead is the manifestation of the physical property that rotating mass discharges energy when decelerated, and charges energy when accelerated.

Second, primary control is a timescale which is the response of generators under the control of governors to increase generation when speed (i.e., frequency) decreases and to decrease generation when speed increases. This is a closed loop local-level control and the typical response time is on the order of seconds. Primary response typically arrests, but does not correct, deviations in frequency. It is the first active layer of defense for grid stability.

Third, secondary control is a timescale in which additional generation set point operation is adjusted to correct for persistent frequency deviation (i.e., drive the frequency back towards 60 Hz). Secondary control timescales are typically minutes.

Fourth, tertiary control is a timescale which is loosely defined as additional dispatcher corrections and scheduling, which typically cover timescales from minutes to hours.

Some embodiments here describe the development of a reduced-order model of the WECC interconnection and the usage of the reduced-order model to estimate the benefit of domestic water heater participation in frequency response (e.g., primary control) to assist in electric grid stability and recovery to large generator outages.

In some embodiments, a simple high-level model of the WECC is developed that can recreate the WECC frequency response to a large generator outage with proper modeling of system inertia, load frequency response, and primary frequency response of generation.

In some embodiments, the model of WECC is then used to estimate the benefit that large-scale water heater energy storage implementation can provide to transient response and system stability.

The embodiments of the disclosure will be understood more fully from the detailed description given below and from the accompanying drawings of the various embodiments of the disclosure, which, however, should not be taken to limit the disclosure to the specific embodiments, but are for explanation and understanding only.

In the following description, numerous details are discussed to provide a more thorough explanation of embodiments of the present disclosure. It will be apparent, however, to one skilled in the art, that embodiments of the present disclosure may be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form, rather than in detail, in order to avoid obscuring embodiments of the present disclosure.

Note that in the corresponding drawings of the embodiments, signals are represented with lines. Some lines may be thicker, to indicate more constituent signal paths, and/or have arrows at one or more ends, to indicate primary information flow direction. Such indications are not intended to be limiting. Rather, the lines are used in connection with one or more exemplary embodiments to facilitate easier understanding of a circuit or a logical unit. Any represented signal, as dictated by design needs or preferences, may actually comprise one or more signals that may travel in either direction and may be implemented with any suitable type of signal scheme.

Throughout the specification, and in the claims, the term “connected” means a direct connection, such as electrical, mechanical, or magnetic connection between the things that are connected, without any intermediary devices. The term “coupled” means a direct or indirect connection, such as a direct electrical, mechanical, or magnetic connection between the things that are connected or an indirect connection, through one or more passive or active intermediary devices.

The term “circuit” or “module” may refer to one or more passive and/or active components that are arranged to cooperate with one another to provide a desired function. The term “signal” may refer to at least one current signal, voltage signal, magnetic signal, or data/clock signal. The meaning of “a,” “an,” and “the” include plural references. The meaning of “in” includes “in” and “on.” The terms “substantially,” “close,” “approximately,” “near,” and “about,” generally refer to being within +/− 10% of a target value.

Unless otherwise specified the use of the ordinal adjectives “first,” “second,” and “third,” etc., to describe a common object, merely indicate that different instances of like objects are being referred to, and are not intended to imply that the objects so described must be in a given sequence, either temporally, spatially, in ranking or in any other manner.

For the purposes of the present disclosure, phrases “A and/or B” and “A or B” mean (A), (B), or (A and B). For the purposes of the present disclosure, the phrase “A, B, and/or C” means (A), (B), (C), (A and B), (A and C), (B and C), or (A, B and C). The term “right,” “front,” “back,” “top” “bottom,” “over,” “under,” and the like in the description and in the claims, if any, are used for descriptive purposes and not necessarily for describing permanent relative positions.

For purposes of the embodiments, the transistors in various circuits and logic blocks described here are metal oxide semiconductor (MOS) transistors or their derivatives, where the MOS transistors include drain, source, gate, and bulk terminals. The transistors and/or the MOS transistor derivatives also include Tri-Gate and FinFET transistors, Gate All Around Cylindrical Transistors, Tunneling FET (TFET), Square Wire, or Rectangular Ribbon Transistors, ferroelectric FET (FeFETs), or other devices implementing transistor functionality like carbon nanotubes or spintronic devices. However, the embodiments are not limited to these transistor types. Other types of transistors that can perform the various functions described here can also be used.

FIG. 1 illustrates plot 100 showing a transition drop in frequency for a Western Grid Interconnect (WECC) model. Here, x-axis is time (in seconds) and y-axis is frequency (in Hertz (Hz)). For purposes of describing the various embodiments, data for plot 100 comes primarily from the 2011 California Independent System Operator frequency response study, specifically the winter low-load high-wind case. The waveform 101 is a base case which is for a total WECC load of 91,300 MW, with wind providing 14% of the electric power generation. Here, a loss of two Palo Verde units totaling 2,690 MW is simulated. This causes a transient drop in frequency around 10 seconds. The frequency nadir is 59.67 Hz at a time of 9.8 seconds after the loss of the Palo Verde units with a settling frequency of 59.78 Hz.

FIG. 2 illustrates reduced-order model 200 of WECC which includes system inertia, load frequency response, and primary response of three classes of generators: slow, medium, and fast. These generators have a wide range of response times, from only seconds to well over a minute to respond to the Palo Verde unit loss.

Reduced-order model 200 comprises power model 201 to provide power disturbance 201a (Pdisturbance); a subtraction unit 202, an attenuator 203 to attenuate by ‘M’ (e.g., 1/M); integrator or frequency converter 204 (e.g., 1/s) to provide per unit frequency 204a (e.g., running time integral of the input); bias generators including: load bias generator 205 (loadBias), fast bias generator 206 (genBiasFast), medium bias generator 207 (genBiasMed), and slow bias generator 208 (genBiasSlow); load response transfer function block 209 (load_resp=1/(Tload.s+1)); fast bias generation response transfer function block 210 (genf_resp=1/(Tgenf.s+1)); medium bias generation response transfer function block 211 (genm_resp=1/(Tgenm.s+1)); slow bias generation response transfer function block 212 (gens_resp 1/(Tgens.s+1)); and summer 213. [

The output of block 209 is load power 209a (Pload) which is received by subtraction unit 202. The output of block 210 is fast generation power 210a (Pgenfast) which is received by summer 213. The output of block 211 is medium generation power 211a (Pgenmed) which is received by summer 213. The output of block 212 is slow generation power 212a (Pgenslow) which is received by summer 213. The output 213a of summer 213 is received by subtraction unit 202.

In some embodiments, the model parameters the various blocks of FIG. 2 are tuned by successive sweeps, until a frequency response fit of a certain threshold is found (e.g., 0.01 Hz RMSE (Root-Mean-Square Error or Deviation)). Model parameters for some embodiments are given in Table I.

TABLE I
MODEL PARAMETERS
Symbol	Name	Value	Units
M	Inertia	15	PU pwr-s/PU freq
loadBias	Load bias	1.027	PU pwr/PU freq
genBiasFast	Fast gen. bias	3.432	PU pwr/PU freq
genBiasMed	Medium gen. bias	2.112	PU pwr/PU freq
genBiasSlow	Slow gen. bias	1.056	PU pwr/PU freq
Tload	Load resp. time const.	0	seconds
Tgenf	Fast gen. time const.	0.5	seconds
Tgenm	Med. gen. time const.	19	seconds
Tgens	Slow gen. time const.	27	seconds
Pbase	Sim. power base	92	GW

FIG. 3 illustrates plot 300 showing a comparison of the base case frequency response and the reduced-order WECC model. Here, x-axis is time (in seconds) and y-axis is frequency (in Hertz (Hz)). Waveform 301 is the same as waveform 101 (e.g., the base case frequency response) while waveform 302 is the transient response of the WECC model of FIG. 2.

FIG. 4 illustrates plot 400 showing response of a load for the three classes of generators to a simulated loss of generation. Here, x-axis is time (in seconds) and y-axis is power (in Giga Watts (GW)). Plot 400 illustrates response of the lost generation (waveform 401), response of the fast generator (waveform 402), response of the slow generator (waveform 403), response of the medium generator (waveform 404), total response of the fast, slow and medium generators (waveform 405), response of the load (waveform 406), and response of the base case (waveform 407).

Here, the term “lost generation” generally refers to an amount generation that went away when the simulated Palo Verde generation units failed. For example, if a 100 MW generator failed and tripped offline, which would be 100 MW of lost generation. In other words, it is the amount of generation that must be made up from other sources to stabilize the grid.

In this example, the fastest generator group (waveform 402) reaches its maximum response around 10 seconds, closely following the frequency of the system. The slow speed and medium speed generator groups (waveforms 403 and 404, respectively) reach their maximum generation in 40 seconds and 60 seconds after the disturbance, respectively. The total generation (waveform 405) is the sum of these three groups and very closely matches the base case generation response (waveform 407).

FIG. 5 illustrates water heater 500 with upper and lower elements. The water heater 500 comprises tank 501, cold water inlet 502, hot water outlet 503, upper thermostat 504, upper heating element 505, lower thermostat 506, and lower heating element 507. Electric water heaters are a significant energy storage resource. Standard residential electric water heaters are 50 or 60 gallons capacity, with one heating element at the bottom of the tank, and another approximately two-thirds up the tank.

The hot water outlet 503 is at the top of tank 501. Upper element (or upper heating element) 505 has priority and heats the upper one-third of tank 501 to the desired temperature, and possibly as high as 140 degrees Fahrenheit. Lower element (or lower heating element) 507 can be active when upper element 505 is off (e.g., when the upper one-third of tank 501 is at the desired temperature) and heats the lower two-thirds of tank 501 to the desired temperature. Generally, the water stays well stratified, and the lower two-thirds of tank 501 can be assumed to hold a temperature below that of the upper one-third of tank 501. This structure allows for great opportunities for utilizing electric water heaters as energy storage. In some embodiments, the temperature of the lower two-thirds of tank 501 can be manipulated in an intelligent way without greatly affecting the temperature of the water in the upper one third of tank 501, which is the water withdrawn by the user.

Assuming a standard residential electric water heater of 50 gallons capacity, an outlet temperature of 130 degrees Fahrenheit, and an inlet temperature of 60 degrees, the lower two-thirds of tank 501 may require approximately 5 kWh of energy to heat. Therefore, by controlling the lower two-thirds of the tank temperature to range from the inlet temperature as the lower bound, and the outlet temperature as the upper bound, a potential energy storage capacity of 5 kWh can be utilized.

Typically, each of upper and lower elements 505 and 507, respectively, are rated for 4.5 kW. Under the standard configuration, upper element 505 has priority to heat the upper portion of tank 501. If upper element 505 is off, lower element 507 is enabled, in accordance with some embodiments. Therefore, the two elements (505 and 507) operate exclusively, and the total water heater power capacity is 4.5 kW, for example. When referring to water heaters generating (e.g., discharging), it may not mean they push energy on the electric grid, but instead decrease their load by the amount they are effectively generating, as if some local generation source has come online to decrease the net load on the electric grid.

In some embodiments, the lower thermostat 506 is replaced with a low-cost embedded controller and power converter. In some embodiments, the low-cost embedded controller and power converter includes a temperature sensor. In some embodiments, the low-cost embedded controller and power converter is operable to support the operation of the lower part of tank 501 such that the electrical load of the water heater depends on the frequency of the electrical grid. In some embodiments, the low-cost embedded controller and power converter is a replacement of the standard thermostat in the lower part of the tank.

A single water heater could be modeled as a battery or capacitor. In some embodiments, an aggregate of millions of water heaters are modeled. The aggregate system has a power capacity, and a state of charge (SOC). Contrary to a single battery or capacitor, the power capacity of the aggregate system is a function of the SOC.

For example, consider the SOC of the entire aggregate system to be 0.5 (i.e., half full). In that case, some of the water heaters will be fully charged, some partly charged, and some fully discharged, such that the total state of charge is 0.5. However, the water heaters that are fully charged may not be able to contribute to total charging power capacity, and the water heaters that are fully discharged may not be able to contribute to total discharging power capacity.

Consider the case of the total system SOC at 0.9. In that case, there are many water heaters that are fully charged, a few that are partly charged, and a very few that are fully discharged. Thus the total charging power capacity will be small, and the total discharging capacity will be large. Therefore, for the aggregate system of millions of water heaters, it is expected for the charging and discharging power capacity to be a function of the total aggregate SOC.

FIG. 6 illustrates plot 600 showing available aggregate water heater power capacity as a function of aggregate water heater SOC. Here, x-axis is SOC (e.g., 0, 0.5., 1), and y-axis is power rating P_rated (e.g., power capacity of water heater (wh) P_{wh,totalcapacity} and -P_{wh,totalcapacity}). For purposes of simplicity, a linear relationship of power capacity to SOC is assumed. For a given SOC, line 601 is the maximum charging power available (e.g., positive power P_charge), and line 602 is the maximum discharging power (e.g., negative power P_discharge) available. When the aggregate SOC is zero, the lower tanks of all the water heaters are at their lowest temperature, and all are available for charging, in accordance with some embodiments. When the aggregate SOC is one, the lower tanks of all the water heaters are at their highest temperature, and all are available for discharging, in accordance with some embodiments. When the aggregate SOC is 0.5, it is approximated that half of the water heaters are available for charging, and half are available for discharging, in accordance with some embodiments.

FIG. 7 illustrates a simplified WECC model 700 with aggregate water heater model, in accordance with some embodiments. WECC model 700 comprises reduced-order model 200, block 701 (which models PwhTotCap*60/c_Hz/Pbase), block 702 (which models PwhTotCap/Pbase), block 703 (which models PwhTotCap/Pbase), first summer block 704, block 705 (which is a saturation block which caps the incoming signal ‘u’ to the limits set by the inputs “Up” and “Lo”), block 706 (which models PwhTotCap/Pbase), block 707 (which models a zero), second summer block 708, block 709 (which denotes running time integration 1/s), and block 710 (which models Pbase/(JwhTotCap*3600) coupled together as shown. Block 705 models the aggregate water heater characteristics illustrated by FIG. 6, in accordance with some embodiments. Referring back to FIG. 7, here, output 701a of block 701 is Pwh_cmd. Output 704a of first summer block 704 is PchargeCapacity. Output 708a of block 708 is PdischargeCapacity. Output 705a of block 705 is Pwh, and output of block 710 is SOC.

The water heater parameters (e.g., PwhTotCap, c_Hz, Pbase, PwhTotCap, Pbase, 1/s, JwhTotCap) are given in Table II. The blocks (e.g., 701-710) in the upper right of FIG. 7 model the water heater power limits, which are linear functions of SOC illustrated in FIG. 6. The commanded water heater power “Pwh_cmd” from block 701 is the desired water heater power above or below the nominal natural water heater load at the moment. The amount of increase or decrease in the water heater load is dictated by the control variable “c_Hz”, which specifies the amount of commanded water heater power per deviation in frequency.

In some embodiments, the goal is to show the benefit to transient response that water heater loads can provide. In some embodiments, the water heater power is to be controlled proportional to frequency, thus providing primary frequency response. The frequency may then drop below a threshold (e.g., 60 Hz) due to the simulated loss of the Palo Verde units (e.g., 2.69 GW=0.029 PU), some of the water heater capacity may respond by decreasing load, thus helping to stabilize the system.

In some embodiments, for a water heater control system, the lower element 507 of the water heater is replaced by an augmented heating element that includes a circuit to detect frequency of the electrical grid. In some embodiments, as the frequency of the electrical grid decreases, the effective element resistance—and therefore the element power—can be effectively changed by pulse width modulating (PWM) the applied voltage. If the home resident is in need of hot water, upper element 505 will be on and lower element 507 is unpowered. In some embodiments, if the upper tank is standing by ready with the desired water temperature, lower element 507 will be available, and thus with a simple augmentation of the thermostat to respond to frequency, will be silently increasing or decreasing the temperature in the lower tank according to the frequency of the electrical grid.

By staying within this traditional water heater control framework, the upper tank and home resident comfort remains the priority. (It is noted, however, that deep discharging of the SOC—i.e., a low temperature of the lower tank—could overwhelm the ability of the upper element to keep the outlet water at the desired temperature).

In some embodiments, this modification may require no additional communication interface. For example, the modification would effectively utilize frequency to detect the state of the generation-load balance. Under this scheme, in some cases, the response of any one individual water heater cannot be guaranteed. However, the aggregate water heater behavior of millions of water heaters is predictable, in accordance with some embodiments.

It is estimated there are approximately 3 to 4 million residential water heaters in the Pacific Northwest of the United States. Assuming each water heater to have a power capacity of 4.5 kW, there is a total residential water heater load capacity of approximately 15,000 MW. For purposes of simplicity, here a very conservative estimate of 3% is made of that resource (e.g., 500 MW) to be available for demand response (e.g., primary frequency participation) for the entire WECC. Assuming each water heater to have a power capacity of 4.5 kW, that is a total of 111,000 water heaters, and further assuming 5 kWh per water heater, there is a total energy storage resource of 555 MWh.

Here, two scenarios are simulated: First, the aggregate water heater load responding at the standard droop of 5%, and Second a more aggressive scenario of the aggregate water heater load responding at a droop of 0.55%. For the 5% droop case—which is a standard setting for generation on primary frequency control—the generator governor is set to modify the generator power set point at a rate of Prada per 3 Hz. For the 0.55% case, the generation set point is modified at a rate of Prada per 0.33 Hz. Generally,

$\begin{array}{cc}P\left(f\right)=P_{\mathrm{setpoint}}+P_{\mathrm{rated}}\frac{f}{c_{\mathrm{Hz}}}& \left(1\right)\end{array}$

For the water heater, control P_setpoint is simply the current natural aggregate water heater load, which is treated as zero in the simulation. (Note that zero load in the simulation does not represent objectively zero Watts, but is instead the bias point of the simulation.) The parameter c_Hz (or c_Hz) is 3 Hz for the 5% case, and 0.33 Hz for the 0.55% case. A value of 0.33 Hz is chosen in this example as that is the frequency deviation nadir of the benchmark case. Thus when cHz is 3Hz, approximately 0:33/3=11% of the total aggregate water heater capacity will be utilized, whereas when cHz is 0.33 Hz, approximately 100% will be utilized.

The aggregate water heater parameters are given in Table II.

TABLE II
WATER HEATER PARAMETERS
Symbol	Name	Value	Units
PwhTotCap	Total aggregate water heater	500	MW
	power capacity
JwhTotCap	Total aggregate water heater	555	MWh
	energy capacity
c_Hz	Water heater freq. response	{0.33, 3.00}	Hz
	constant

FIG. 8 illustrates plot 800 showing a frequency response of the WECC model of FIG. 7 to a Palo Verde outage under three difference scenarios. It is pointed out that those elements of FIG. 8 having the same reference numbers (or names) as the elements of any other figure can operate or function in any manner similar to that described, but are not limited to such. Here, x-axis is time (in seconds) and y-axis is frequency (in Hertz). Plot 800 shows the frequency response of the WECC to the Palo Verde outage (e.g., 2.69 GW of lost generation) under three scenarios. The first scenario is the base case of no water heater energy storage which is illustrated by waveform 801. The second scenario is the standard case of all available water heaters responding for 3 Hz (e.g., 5%) deviation, which is illustrated by waveform 802. The third scenario is the aggressive case of all available water heaters responding for a 0.33 Hz (e.g., 0.55%) deviation, which is illustrated by waveform 803. The frequency nadirs and settling times are summarized in Table III.

TABLE III
COMPARISON OF FREQ. RESP. WITH
WATER HEATERS PARTICIPATING
	Case	Freq nadir	Settling freq
	Base	59.666 Hz	59.768 Hz
	c_Hz = 3 Hz	59.673 Hz	59.771 Hz
	c_Hz = 0.33 Hz	59.697 Hz	59.789 Hz

The initial aggregate water heater SOC is 0.5, and therefore 250 MW of the 500 MW is available for charging, and 250 MW is available for discharging. The results show that for the standard case of full water heater response per 3 Hz of frequency deviation as shown by waveform 802, only about ⅕ (e.g., 50 MW) of the available 250 MW is utilized. However, even with this modest response, there is a small but measurable benefit in the frequency nadir and settling frequency. For the more aggressive case of full water heater response per 0.33 Hz of frequency deviation as shown by waveform 803, the full water heater capacity of 250 MW is utilized. The benefit is much greater in this case, with the frequency nadir increased from 59.666 Hz to 59.697 Hz over the base case (which is waveform 801), which is a 10% improvement in the deviation from 60 Hz. The SOC is not plotted, but in this example it is effectively unchanged over the 60 second simulation time span as the energy storage capacity of 555 MWh is so large.

FIG. 9 illustrates plot 900 showing water heater power in response to change in frequency due to loss of generation for two cases. Here, x-axis is time (in seconds) and y-axis is Power (in MW). Plot 900 shows water heater power in response to the change in frequency due to the loss of 2.69 GW of generation for two cases. The first case is illustrated by waveform 901 which is the case for all available water heater power responding per 3 Hz frequency deviation. The second case is illustrated by waveform 902 which is the case for all available water heater power responding per 0.33 Hz deviation.

FIG. 10A illustrates a water heater 1000 (also referred to as an energy storing apparatus) with power electronics and control, according to some embodiments of the disclosure. It is pointed out that those elements of FIG. 10A having the same reference numbers (or names) as the elements of any other figure can operate or function in any manner similar to that described, but are not limited to such. FIG. 10A is described with reference to other figures including FIG. 5.

Standard electric hot water heater control relies on chained operation of the upper tank and lower tank relays. Here, the upper tank is also referred to as the first region while the lower tank is referred to as the second region, where the first region is thermally coupled to the second region. The upper tank thermostat/relay 1001/504 has priority, as the hot water is drawn from the upper tank. If the upper tank temperature is below its lower limit, the upper tank relay is closed (e.g., connected) and the upper tank heating element is energized, and the lower element is off. If the upper tank temperature is above its upper limit, the upper tank element is de-energized, and the lower thermostat is active. If the lower thermostat is active, if the lower tank temperature is below its lower limit, the lower element is energized. If the lower tank temperature is above its upper limit, the lower element is de-energized. In this standard mode of operation, the upper tank heating has priority, and merely one of the two 4.5 kW heating elements is active at a time. In some embodiments, the first region has set point which is set to a temperature lower than a temperature of the second region.

In some embodiments, the operation of upper thermostat/relay 1001/504 is unchanged. The upper thermostat/relay 1001/504 comprises thermostat 1001a coupled to 220-240 V input 1009 via wires 1005 and 1006. Wires 1007 and 1008 are coupled to upper thermostat/relay 1001/504 and lower thermostat/relay 1003/506. The upper thermostat/relay 504 is coupled to the upper heater element 1002/505. The lower thermostat/relay 1003/506 is coupled to the lower heater element 1004/507. The lower thermostat/relay 506 comprises the thermostat 1003a. In some embodiments, the lower thermostat/relay 1006/506 includes a “smart” component of a same physical form-factor as a traditional lower thermostat.

FIG. 10B illustrates apparatus 1020/1003/506 for the lower thermostat, according to some embodiments of the disclosure. It is pointed out that those elements of FIG. 10B having the same reference numbers (or names) as the elements of any other figure can operate or function in any manner similar to that described, but are not limited to such. In some embodiments apparatus 1020/1003 comprises micro-controller 1021 (or controller), a power-converter 1022 that controls the amount of power applied to lower element 507, and temperature sensor 1023 coupled together as shown. In some embodiments, micro-controller 1021 is one of: A Digital Signal Processor (DSP), an Application Specific Integrated Circuit (ASCI), a general purpose Central Processing Unit (CPU), or a low power logic implementing a simple finite state machine to perform the various methods described here. In some embodiments, power-converter 1022 is to control the amount of power applied to the first region. In some embodiments, power-converter 1022 is a buck converter or any other DC-DC converter. In some embodiments, power-converter 1022 is operable to apply power to the first region according to temperature sensed by temperature sensor 1023 and a frequency of the electric grid.

In some embodiments, the power applied to lower element 507 is applied based on two considerations: temperature and grid frequency. In some embodiments, the two proportional loops (first loop and second loop) regulating temperature and grid-frequency are operated in parallel. In some embodiments, the first proportional loop is to regulate a temperature sensed by temperature sensor 1023. In some embodiments, the second proportional loop regulates rate-of-change of grid frequency to emulate inertia of the electric grid.

For example, the second proportional loop adjusts a frequency of the electric grid. In some embodiments, the first and second proportional loops operate in parallel. In some embodiments, the lower tank temperature set-point is set to a temperature slightly lower than the upper tank, to allow for range of operation above the set-point when the grid frequency is low, without exceeding the tank maximum temperature. In some embodiments, when the electric grid frequency is high, it will tend to depress the lower tank temperature, until the frequency-regulating loop reaches equilibrium with the temperature regulating loop.

FIGS. 11A-B illustrates plots 1100 and 1120 showing power and frequency simulation results of 1000 water heaters over a 24-hour period. For plot 1100, the x-axis is time in hours while the y-axis is power in kW. Trace 1101 is the aggregate load power of water heaters without the lower thermostat replacement (e.g., using a traditional lower thermostat). Trace 1102 is the aggregate load power of the water heaters with lower thermostat 506. For plot 1120, the x-axis is time in hours while the y-axis is power in kW. Trace 1121 is the grid frequency. Several features are exhibited by these traces. For example, at approximately 6 hours, there is a large grid frequency disturbance. This disturbance causes a nearly instantaneous stabilizing response in the water heater load. From hour 16 to hour 20 along the x-axis, the grid frequency is slightly below 60 Hz, which causes a corresponding decrease in the aggregate water load of the demand response enabled water heaters. From hour 22 to hour 24 along the x-axis, the grid frequency is above 60 Hz, and the demand response enabled water heater load is large. In hours 16-20, while the frequency is slightly below 60 Hz, waveform 1102 increases in that period, at least over the first two hours. It goes from about 500 kW at approximately 16 hours to approximately 750 kW by about 18 hours. At 22 hours, frequency ticks up slightly, and then the demand response decrease from about 600 kW to below 400 kW over that time period.

FIG. 12 illustrates water heater 1200 and operation of its thermostats, according to some embodiments of the disclosure. It is pointed out that those elements of FIG. 12 having the same reference numbers (or names) as the elements of any other figure can operate or function in any manner similar to that described, but are not limited to such. Water heater 1200 is same as water heater 1000. Here, upper thermostat block 504 comprises relay 1201 and thermostat 1203 coupled via wire 1202. An interface to the grid is also provided. Here, lower thermostat block 506 comprises thermostat 1204 coupled to lower element 507 via wires 1206 and 1207. An interface to the electric grid is also provided. Plot 1209 shows hysteresis between On and Off times for lower thermostat 506. Plot 1210 shows hysteresis between On (curve 1210a) and Off (curve 1210b) times for upper thermostat 504.

FIG. 13 illustrates circuit level architecture 1300 of the power electronics and control for lower thermostat 506, according to some embodiments of the disclosure. It is pointed out that those elements of FIG. 13 having the same reference numbers (or names) as the elements of any other figure can operate or function in any manner similar to that described, but are not limited to such

In some embodiments, architecture 1300 comprises diode bridge rectifier 1301, driver 1302, and control block 1303. In some embodiments, control block 1303 switches between T* and T based on frequency ‘f’. The output of control block 1303 is coupled to driver 1302 which provides control signals for controlling gate terminals of transistors in circuit 1301. In some embodiments, circuit block 1301 comprises inductor capacitor ‘C’, first n-type transistor MN1, second n-type transistor MN2, and diodes D1, D2, D3, and D4 coupled together as shown. Here, inductor L, capacitor C, and transistors MN1 and MN2 together form an analog portion of a buck converter or power converter. The diodes D1, D2, D3, and D4 perform the function of a rectifier.

In some embodiments, inductor ‘L’ and capacitor ‘C’ are coupled to lower heating element 507 via nodes 1206 and 1207. One end of inductor ‘L’ is coupled to node n1 which is a common node coupling transistors MN1 and MN2. The gate terminal of MN1 is controlled by “g2” provided by driver 1302. The gate terminal of MN2 is controlled by g1 (or V₁) provided by driver 1302. Diodes D1 and D2 are coupled between supply Vdd and node 1207. Diodes D3 and D4 are coupled between supply Vdd and node 1207. The node coupling diodes D1 and D2 is coupled to the electric grid. The node coupling diodes D3 and D4 is coupled to the electric grid. The transfer function of the inductor and capacitor network is shown by plot 1304, where input is V₁ and output is V₂. The rectified utility voltage (e.g., voltage across Vdd and node 1207) is shown by plot 1305. The transient response at the nodes coupled to the electric grid is given by 1306.

FIG. 14 illustrates architecture 1400 of the lower thermostat, according to some embodiments of the disclosure. It is pointed out that those elements of FIG. 14 having the same reference numbers (or names) as the elements of any other figure can operate or function in any manner similar to that described, but are not limited to such. Architecture 1400 illustrates the apparatus that inputs to control block 1303. In some embodiments, architecture 1400 includes step down converter (or transformer) 1401, analog-to-digital converter (ADC) 1402, phase locked loop (PLL) 1403, adder/subtractor 1404, gain stage 1405 (e.g., having gain K₁), saturation block 1406 adder/subtractor 1407, gain stage 1408 (e.g., having gain K₂), adder/subtractor 1409, and sampler 1411 coupled together as shown. Saturation block 1406 limit the range of the desired temperatures (T*) to a safe upper and lower limit, in accordance with some embodiments. Saturation block 1406 may also limit “D*” to between 0 and 1, in accordance with some embodiments.

In some embodiments, an antenna (not shown) is provided to send and receive information associated with apparatus 1400. For example, various control parameters can be read, set or adjusted by wireless means using the antenna. In some embodiments, the antenna may comprise one or more directional or omnidirectional antennas, including monopole antennas, dipole antennas, loop antennas, patch antennas, microstrip antennas, coplanar wave antennas, or other types of antennas suitable for transmission of Radio Frequency (RF) signals. In some multiple-input multiple-output (MIMO) embodiments, Antenna(s) 101 are separated to take advantage of spatial diversity.

ADCs are apparatuses that convert continuous physical quantities (e.g., voltages) to digital numbers that represent the amplitude of the physical quantities. In some embodiments, ADC 1402 converts the analog output of step down converter 1401 to its corresponding digital representation. Any suitable ADC may be used to implement ADC 1402. For example, ADC 1402 is one of: direct-conversion ADC (for flash ADC), two-step flash ADC, successive-approximation ADC (SAR ADC), ramp-compare ADC, Wilkinson ADC, integrating ADC, delta-encoded ADC or counter-ramp, pipeline ADC (also called subranging quantizer), sigma-delta ADC (also known as a delta-sigma ADC), time-interleaved ADC, ADC with intermediate FM stage, or time-stretch ADC. For purposes of explaining the various embodiments, ADC 1402 is considered to be flash ADC.

In some embodiments, PLL 1403 is one of: an analog PLL, digital PLL, mixed signal PLL (e.g., having analog and digital components), Inductor-capacitor tank (LC) PLL, self-biased PLL, etc.

FIGS. 15A-B illustrate flowcharts of a method operation of the water heater, according to some embodiments of the disclosure. It is pointed out that those elements of FIGS. 15A-B having the same reference numbers (or names) as the elements of any other figure can operate or function in any manner similar to that described, but are not limited to such.

Although the blocks in the flowchart with reference to FIGS. 15A-B are shown in a particular order, the order of the actions can be modified. Thus, the illustrated embodiments can be performed in a different order, and some actions/blocks may be performed in parallel. Some of the blocks and/or operations listed in FIGS. 15A-B are optional in accordance with certain embodiments. The numbering of the blocks presented is for the sake of clarity and is not intended to prescribe an order of operations in which the various blocks must occur. Additionally, operations from the various flows may be utilized in a variety of combinations.

The process begins to at block 1501. At block 1501, a determination is made whether upper relay direction flag is positive or negative. In some embodiments, the upper relay direction flag is used to model the operation of the upper standard water heater relay. The relay has a hysteresis characteristic. For example, if the relay is on, it turns off at a different point that it would turn on if it were off as shown by 1210 of FIG. 12. The upper relay direction flag tracks whether the relay is at curve 1210a or curve 1210b of plot 1210.

Referring back to FIGS. 15A-B, if upper relay direction flag is positive, then the process proceeds to block 1502. At block 1502, a determination is made whether the upper tank temperature (temp) is greater than upper tank upper temperature limit. If the upper tank temperature is greater than the upper tank upper temperature limit, the process proceeds to block 1507. At block 1507, zero power (i.e., no power) is applied to upper element 505 and upper relay direction flag is set negative. The process then proceeds to block 1508. If upper relay direction flag is negative, then the process proceeds to block 1503.

At block 1503, a determination is made whether the upper tank temperature is less than the upper tank lower temperature limit. If the upper tank temperature is less than the upper tank lower temperature limit, the process proceeds to block 1504. At block 1504, power is applied to upper element 505 (while zero power or no power is applied to lower element 507). The upper relay direction flag is then set positive. The process them proceeds to block 1501.

If the upper tank temperature is greater than or equal to the upper tank lower temperature limit, the process proceeds to block 1506. At block 1506, zero power (or no power) is applied to upper element 505. The process them proceeds to block 1508. At block 1508, a determination is made whether the lower tank temperature is greater than the lower tank temperature limit. If the lower tank temperature is greater than the lower tank temperature limit, the process proceeds to block 1509. At block 1509, zero power (or no power) is applied lower element 507. The process then proceeds to block 1501. If the upper tank temperature is less than or equal to the upper tank upper temperature limit, the process proceeds to block 1505. At block 1505, power is applied to the upper element 505 while zero power is applied to lower element 507. The process then proceeds to block 1501.

If the lower tank temperature is less than or equal to the lower tank temperature limit, the process proceeds to block 1510 as indicated by reference sign ‘A’. At block 1510, parameter ‘A’ is calculated as K₁ *(lower temperature set point—lower tank temperature), where K₁ is the gain of block 1405 of FIG. 14. Referring back to FIG. 15B, the process then proceeds to block 1511. At block 1511, parameter ‘B’ is calculated as K₂*(grid frequency-grid frequency setpoint), where K₂ is the gain of block 1408 of FIG. 14. Referring back to FIG. 15B, the process then proceeds to block 1512. At block 1512, a determination is made whether a sum of parameters ‘A’ and ‘B’ (i.e., A+B) is greater than maximum lower element power rating. If the sum of parameters ‘A’ and ‘B’ is greater than maximum lower element power rating, the process proceeds to block 1513.

At block 1513, maximum rated power is applied to lower element 507. The process then proceeds to block 1501 as indicated by reference sign ‘B’. If the sum of parameters ‘A’ and ‘B’ is less than or equal to the maximum lower element power rating, the process proceeds to block 1514. At block 1514, a determination is made whether the sum of parameters of ‘A’ and ‘B’ is less than zero. If the sum of parameters ‘A’ and ‘B’ is less than zero, the process proceeds to block 1515. At block 1515, zero rated power is applied to lower element 507. The process then proceeds to block 1501 as indicated by reference sign ‘B’. If the sum of parameters ‘A’ and ‘B’ is greater than or equal to zero, the process proceeds to block 1516. At block 1516, power proportional (or equal) to the sum of ‘A’ and ‘B’ is applied to lower element 507. The process then proceeds to block 1501 as indicated by reference sign ‘B’.

Elements of embodiments are also provided as a machine-readable medium for storing the computer-executable instructions. The machine-readable medium may include, but is not limited to, flash memory, optical disks, CD-ROMs, DVD ROMs, RAMs, EPROMs, EEPROMs, magnetic or optical cards, phase change memory (PCM), or other types of machine-readable media suitable for storing electronic or computer-executable instructions. For example, embodiments of the disclosure may be downloaded as a computer program (e.g., BIOS) which may be transferred from a remote computer (e.g., a server) to a requesting computer (e.g., a client) by way of data signals via a communication link (e.g., a modem or network connection).

Reference in the specification to “an embodiment,” “one embodiment,” “some embodiments,” or “other embodiments” means that a particular feature, structure, or characteristic described in connection with the embodiments is included in at least some embodiments, but not necessarily all embodiments. The various appearances of “an embodiment,” “one embodiment,” or “some embodiments” are not necessarily all referring to the same embodiments. If the specification states a component, feature, structure, or characteristic “may,” “might,” or “could” be included, that particular component, feature, structure, or characteristic is not required to be included. If the specification or claim refers to “a” or “an” element, that does not mean there is only one of the elements. If the specification or claims refer to “an additional” element, that does not preclude there being more than one of the additional element.

Furthermore, the particular features, structures, functions, or characteristics may be combined in any suitable manner in one or more embodiments. For example, a first embodiment may be combined with a second embodiment anywhere the particular features, structures, functions, or characteristics associated with the two embodiments are not mutually exclusive.

While the disclosure has been described in conjunction with specific embodiments thereof, many alternatives, modifications and variations of such embodiments will be apparent to those of ordinary skill in the art in light of the foregoing description. The embodiments of the disclosure are intended to embrace all such alternatives, modifications, and variations as to fall within the broad scope of the appended claims.

In addition, well known power/ground connections to integrated circuit (IC) chips and other components may or may not be shown within the presented figures, for simplicity of illustration and discussion, and so as not to obscure the disclosure. Further, arrangements may be shown in block diagram form in order to avoid obscuring the disclosure, and also in view of the fact that specifics with respect to implementation of such block diagram arrangements are highly dependent upon the platform within which the present disclosure is to be implemented (i.e., such specifics should be well within purview of one skilled in the art). Where specific details (e.g., circuits) are set forth in order to describe example embodiments of the disclosure, it should be apparent to one skilled in the art that the disclosure can be practiced without, or with variation of, these specific details. The description is thus to be regarded as illustrative instead of limiting.

The following examples pertain to further embodiments. Specifics in the examples may be used anywhere in one or more embodiments. All optional features of the apparatus described herein may also be implemented with respect to a method or process.

For example, an energy storing apparatus is provided which comprises: a first region having a thermostat which includes: a temperature sensor; a controller coupled to the temperature sensor; and a power-converter coupled to the controller; and a second region thermally coupled to the first region. In some embodiments, the power-converter is to control the amount of power applied to the first region. In some embodiments, the power-converter is operable to apply power to the first region according to temperature sensed by the temperature sensor and a frequency of an electric grid. In some embodiments, the energy storing apparatus comprises: a first proportional loop to regulate a temperature sensed by the temperature sensor; and a second proportional loop to adjust frequency of an electric grid. In some embodiments, the first and second proportional loops to operate in parallel. In some embodiments, the second proportional loop is to emulate inertia of the electric grid. In some embodiments, the first region has set point which is set to a temperature lower than a temperature of the second region,

In another example, a method for controlling frequency of an electric grid is provided. In some embodiments, the method comprises: determining whether an upper relay direction flag is positive or negative, and if negative, determining whether an upper tank temperature is less than an upper tank lower temperature limit, and if positive, determining whether an upper tank temperature is greater than an upper tank upper temperature limit; and applying power to an upper heating element, setting the upper relay direction flag to positive, and applying zero power to a lower heating element in response to determining that the upper tank temperature is less than the upper tank lower temperature limit, wherein the upper and lower heating elements are part of a water heater. In some embodiments, the method comprises: applying zero power to the upper heating element in response to determining that the upper tank temperature is greater than or equal to the upper tank lower temperature limit.

In some embodiments, the method comprises: applying zero power to the upper heating element and setting upper relay direction flag to negative in response to determining that the upper tank temperature is greater than the upper tank upper temperature limit; or applying power to the upper heating element and applying no power to the lower heating element in response to determining that the upper tank temperature is less than or equal to the upper tank upper temperature limit. In some embodiments, the method comprises determining whether a lower tank temperature is greater than a lower tank temperature limit. In some embodiments, the method comprises: applying zero power to the lower heating element in response to determining that the lower tank temperature is greater than the lower tank temperature limit; or computing first and second parameters in response to determining that the lower tank temperature is less than or equal to the lower tank temperature limit, wherein the first parameter is a function of the lower tank temperature and a lower temperature set point, and wherein the second parameter is a function of the frequency of the electric grid and a frequency set point of the electric grid.

In some embodiments, the method comprises: determining whether a sum of the first and second parameters is greater than a maximum lower heating element power rating; and applying a maximum rated power to the lower heating element in response to determining that the sum of the first and second parameters is greater than the maximum lower heating element power rating. In some embodiments, the method comprises: determining whether a sum of the first and second parameters is less than zero; applying zero rated power to the lower heating element in response to determining that the sum of the first and second parameters is less than zero; or applying power to the lower element in response to determining that the sum of the first and second parameters is greater than or equal to zero, wherein the applied power is a function of the sum of the first and second parameters.

In another example, an apparatus is provided which comprises: an interface to be coupled to an electric grid; a temperature sensor; a controller coupled to the temperature sensor; and a power-converter coupled to the controller, wherein the power-converter is to couple via passive devices to a heating element, wherein the power-converter is to apply power to the heating element according to a temperature sensed by the temperature sensor and a frequency of the electric grid. In some embodiments, the apparatus comprises at least two diodes coupled to the interface. In some embodiments, the power-converter includes at least two transistors coupled in series such than a common node of the least two transistors is coupled to at least one of the passive devices, and wherein the power-converter is to control an amount of power applied to the heating element.

In some embodiments, the apparatus comprises: a first proportional loop to regulate a temperature sensed by the temperature sensor; and a second proportional loop to adjust frequency of an electric grid. In some embodiments, the first and second proportional loops are to operate in parallel. In some embodiments, the second proportional loop is to emulate inertia of the electric grid.

An abstract is provided that will allow the reader to ascertain the nature and gist of the technical disclosure. The abstract is submitted with the understanding that it will not be used to limit the scope or meaning of the claims. The following claims are hereby incorporated into the detailed description, with each claim standing on its own as a separate embodiment.

Claims

1. An energy storing apparatus comprising:

a first region having a thermostat which includes: a temperature sensor; a controller coupled to the temperature sensor; and a power-converter coupled to the controller; and

a second region thermally coupled to the first region.

2. The energy storing apparatus of claim 1, wherein the power-converter is to control the amount of power applied to the first region.

3. The energy storing apparatus of claim 2, wherein the power-converter is operable to apply power to the first region according to temperature sensed by the temperature sensor and a frequency of an electric grid.

4. The energy storing apparatus of claim 1 comprises:

a first proportional loop to regulate a temperature sensed by the temperature sensor; and

a second proportional loop to adjust frequency of an electric grid.

5. The energy storing apparatus of claim 4, wherein the first and second proportional loops are to operate in parallel.

6. The energy storing apparatus of claim 4, wherein the second proportional loop is to emulate inertia of the electric grid.

7. The energy storing apparatus of claim 1, wherein the first region has a set point which is set to a temperature lower than a temperature of the second region.

8. A method for controlling frequency of an electric grid, the method comprising:

determining whether an upper relay direction flag is positive or negative, and if negative, determining whether an upper tank temperature is less than

an upper tank lower temperature limit, and if positive, determining whether an upper tank temperature is greater

than an upper tank upper temperature limit; and

applying power to an upper heating element, setting the upper relay direction flag to positive, and applying zero power to a lower heating element in response to determining that the upper tank temperature is less than the upper tank lower temperature limit,

wherein the upper and lower heating elements are part of a water heater.

9. The method of claim 8 comprising: applying zero power to the upper heating element in response to determining that the upper tank temperature is greater than or equal to the upper tank lower temperature limit.

10. The method of claim 9 comprising:

applying zero power to the upper heating element and setting the upper relay direction flag to negative in response to determining that the upper tank temperature is greater than the upper tank upper temperature limit; or

applying power to the upper heating element and applying no power to the lower heating element in response to determining that the upper tank temperature is less than or equal to the upper tank upper temperature limit.

11. The method of claim 10 comprising: determining whether a lower tank temperature is greater than a lower tank temperature limit.

12. The method of claim 11 comprising:

applying zero power to the lower heating element in response to determining that the lower tank temperature is greater than the lower tank temperature limit; or

computing first and second parameters in response to determining that the lower tank temperature is less than or equal to the lower tank temperature limit, wherein the first parameter is a function of the lower tank temperature and a lower temperature set point, and wherein the second parameter is a function of the frequency of the electric grid and a frequency set point of the electric grid.

13. The method of claim 12 comprising:

determining whether a sum of the first and second parameters is greater than a maximum lower heating element power rating; and

applying a maximum rated power to the lower heating element in response to determining that the sum of the first and second parameters is greater than the maximum lower heating element power rating.

14. The method of claim 13 comprising:

determining whether a sum of the first and second parameters is less than zero;

applying zero rated power to the lower heating element in response to determining that the sum of the first and second parameters is less than zero; or

applying power to the lower heating element in response to determining that the sum of the first and second parameters is greater than or equal to zero, wherein the applied power is a function of the sum of the first and second parameters.

15. An apparatus comprising:

an interface to be coupled to an electric grid;

a temperature sensor;

a controller coupled to the temperature sensor; and

a power-converter coupled to the controller, wherein the power-converter is to couple via passive devices to a heating element, wherein the power-converter is to apply power to the heating element according to a temperature sensed by the temperature sensor and a frequency of the electric grid.

16. The apparatus of claim 15 comprises at least two diodes coupled to the interface.

17. The apparatus of claim 15, wherein the power-converter includes at least two transistors coupled in series such than a common node of the least two transistors is coupled to at least one of the passive devices, and wherein the power-converter is to control an amount of power applied to the heating element.

18. The apparatus of claim 15 comprises:

a first proportional loop to regulate a temperature sensed by the temperature sensor; and

a second proportional loop to adjust frequency of an electric grid.

19. The apparatus of claim 18, wherein the first and second proportional loops are to operate in parallel.

20. The apparatus of claim 18, wherein the second proportional loop is to emulate inertia of the electric grid.