CONTROL SYSTEMS FOR LIQUID DESICCANT AIR CONDITIONING SYSTEMS
Methods and control systems are disclosed for operating a liquid desiccant air-conditioning system to efficiently maintain a target temperature and humidity level in a space.
This application claims priority from U.S. Provisional Patent Application No. 62/580,249 filed on Nov. 1, 2017 entitled CONTROL SYSTEMS FOR LIQUID DESICCANT AIR CONDITIONING SYSTEMS, which is hereby incorporated by reference.
STATEMENT AS TO FEDERALLY SPONSORED RESEARCHThe United States Government has rights in this invention under Contract No. DE-AC36-08G028308 between the United States Department of Energy and the Alliance for Sustainable Energy, LLC, the Manager and Operator of the National Renewable Energy Laboratory.
BACKGROUNDThe present application relates generally to the use of liquid desiccants (LD) in combination with heat pumps, compressors, and chillers to condition the temperature and humidity of an air stream entering a space. Current control systems are designed for Direct eXpansion (DX) systems or solid desiccant wheel systems. Liquid desiccant air conditioning systems (LDAC) allow for significant independent control of humidity and temperature, while reducing the energy required to achieve specific supply air targets by up to 50%. The capability to Independently control of temperature and humidity supplied by LDAC systems not only improves comfort and health, but also simplifies building management controls and reduces the risk of humidity damage to the building. Depending on the latent and sensible loads that need to be managed, the system can either heat and simultaneously humidify air, or heat and simultaneously dehumidify or cool the air while humidifying or dehumidifying the air. This enables system managers to maintain more comfortable and healthier indoor air conditions than conventional systems can provide. Such independent control of humidity and temperature is critical for many applications, including but not limited to outside air supply to commercial buildings in monsoon regions or the use of air conditioners in buildings which have spaces with very different sensible heat ratio (SHR) requirements, like grocery stores with high humidity and relatively warm grocery/bakery sections and dry and cool refrigeration sections. Controlling systems and buildings to meet such load characteristics requires appropriate control strategies for individual pieces of equipment and for the complete buildings.
Desiccant dehumidification systems—both liquid and solid desiccants—have been used in parallel to conventional vapor compression HVAC equipment to help reduce humidity in spaces, particularly in spaces that require large amounts of outdoor air or that have large humidity loads inside the building space itself. (ASHRAE 2012 Handbook of HVAC Systems and Equipment, Chapter 24, p. 24.10). Humid climates, such as Miami, Fla., require a lot of energy to properly treat (dehumidify and cool) the fresh air that is required for a space's occupant comfort. Solid desiccant dehumidification systems have been used for many years and are generally quite efficient at removing moisture from the air stream. However, liquid desiccant systems generally use concentrated salt solutions such as ionic solutions of LiCl, LiBr, or CaCl2 and water. Membrane based liquid desiccant systems have been primarily applied to unitary rooftop units for commercial buildings. However, in addition to rooftop units, commercial buildings also use air handlers located inside technical spaces in the building for the cooling and heating of both outside air and recirculated air. There is an additional substantial market for chillers that provide cold water to coils inside the building and use evaporative cooling for high efficiency condensers. Residential and small commercial buildings often use split air conditioners wherein the condenser (together with the compressor and control system) is located outside and one or more evaporator cooling coils is installed in the space than needs to be cooled. In Asia in particular (which is generally hot and humid) the split air conditioning system is the preferred method of cooling (and sometimes heating) a space. Each of those require different configurations and control mechanisms. Various configurations for managing humidity and temperature independently have been disclosed in U.S. Pat. No. 9,243,810 and U.S. Patent Application No. 62/580270. They disclose configurations with liquid desiccant components, sensible coils, direct and indirect evaporative cooling and water addition. They disclose how performance for various modes of operation is optimized, including cooling and dehumidification, cooling only, cooling and humidification, heating only, heating and humidification and heating and dehumidification. Cooling and heating refer here to changing the DB condition only; changes in total enthalpy will be described as net heating and net cooling. Existing control strategies need to be adjusted to optimize efficiency and comfort in each of these operating modes for the different configurations.
Liquid desiccant systems generally have two separate functions. The conditioning side of the system provides conditioning of air to the required conditions, which are typically set using thermostats or humidistats. The regeneration side of the system provides a reconditioning function of the liquid desiccant so that it can be re-used on the conditioning side. Liquid desiccant is typically pumped or moved between the two sides, and a control system helps to ensure that the flows and concentrations of the liquid desiccant is properly balanced between the two sides as conditions necessitate and that excess heat and moisture are properly dealt with, without leading to over-concentrating or under-concentrating of the desiccant.
Performance of the conditioner and regenerator is driven by the flow rates and temperatures the three fluids: air, water, and desiccant in the heat exchangers. The dehumidification potential is driven by the concentration of the desiccant, which can be controlled in a number of ways as disclosed, e.g., in U.S. Pat. No. 9,243,810 and U.S. Patent Application No. 62/580270. The primary controls are compressor power and fan speed and water flow of the regenerator. Adding sensible cooling capacity with additional air-to-water or air-to-refrigerant coils can greatly broaden the amount work done by the system and the range of sensible heat ratios that can be supported by increasing the ability for independent control of temperature and humidity within the unit.
At a building level, overall effectiveness of the system is driven by the mix of systems used and how they are used. For example, the ASHRAE DOAS (Dedicated Outside Air Unit) design guide identifies how conditioning the outside air to handle the complete dehumidification load of the building can increase overall efficiency, due to the much improved efficiency of air cooled coils when only used for sensible cooling,.
The benefits of liquid desiccant systems have been described in various patents, e.g., U.S. Pat. No. 9,243,810 and others. Such systems have been clearly demonstrated for hot and humid climates with a large latent load. As buildings get better insulated, these latent loads increase as a percentage of total cooling loads, making effective dehumidification more important. As internal sensible loads are reduced in tighter, better insulated buildings, conditioning ventilation air becomes an even more significant part of total cooling and heating loads.
Extreme design conditions, including very humid and cool, very hot and dry, and very humid and cold require special cooling and heating solutions for which earlier liquid desiccant systems are not optimized.
At very high temperatures (>100 F) and very low humidity (<20% RH), liquid desiccant systems can't operate efficiently and need special controls to avoid crystallization of the desiccant. Traditional evaporative cooling systems do well at low humidities and moderate cooling requirements, but are unable to deal with extreme heat or with more humid conditions that tend to occur at least part of the time in most locations.
Traditional cooling systems use refrigerant coils that are air cooled and are best suited for sensible cooling. Condense forming on the coil acts as an insulator that reduces its capacity. Thus multiple coils need to be used in series to fully dehumidify and cool the air. Four and six row coils are commonly used. Still, traditional systems often cannot handle the full latent load without significantly overcooling the air and then reheating it, or mixing high volumes of return air with small volumes of outside air to minimize the humidity level of the mix. Especially in times where only a small amount of sensible cooling is required, humidity control is compromised. When compressors are cycled to manage smaller loads, bursts of humidity enter the building as coils warm up and evaporate condense back into the air. Many split systems provide heating by operating as a reversible heat pump system. The liquid desiccant system is able to cool outside air without frost forming, significantly improving system efficiency by reducing or eliminating defrost cycles. The removal of humidity from the outside air also enables the humidification of the heated space, maintaining healthy RH levels between 30-60% RH. These tend to be most useful in moderate climates where cooling and heating loads are roughly in balance. Very cold climates like the Midwest and Northeast of the US still require additional heating, often from natural gas or oil. In more moderate climates, heat pump effectiveness is limited by humidity, which can lead to frost forming and the use of very inefficient defrost cycles. Using a liquid desiccant condenser coil prevents frost forming in a heat pump system.
Liquid desiccants can achieve effective dehumidification at higher temperatures of the compressors evaporator, during the cooling cycle. The regenerator fully rejects the condenser energy at lower temperatures than traditional air cooled systems. As a result, the compressor can move energy from the conditioned space to outside the space at a much lower temperature differential than traditional systems. This improves the efficiency of the compressor in proportion to the reduction in the temperature difference. The lower temperature difference between the condenser and evaporator is the lift of the compressor and drives the efficiency of the combination of compressor-based cooling and heating with liquid desiccant heat exchangers.
As disclosed in U.S. Pat. No. 9,243,810, by actively diluting the desiccant, e.g., by using vapor transfer membrane modules, a liquid desiccant system can increase the ratio of sensible cooling to latent cooling. It can even starts to act like a direct evaporative cooler, which allows it to maintain a target minimum dewpoint (DP) under very dry conditions, without maintaining dry bulb (DB) targets at high wetbulb (WB) conditions, something traditional direct evaporators cannot do. While the conditioner operates at cooler temperatures the overall temperature differential over the compressor tends to be further reduced due to a much larger reduction in condenser temperatures, as diluted desiccant increases evaporation at the condenser transforming it into a de facto water cooled air conditioning system with comparable efficiency but with significantly reduced water consumption.
Existing control strategies for air conditioners can rely on bandwidth control, adaptive control or predictive control, they need to be modified for the various configurations of liquid desiccant systems and then optimized for each of the operating modes described before.
Additional building humidity “guidelines” are being developed to encourage maximum, and sometimes even minimum, humidity levels mostly driven by health considerations, especially the impact on respiratory disease and allergies.
In dry climates, water cooled chillers and evaporative coolers use the evaporation energy of water to cool spaces and/or improve compressor efficiency, but this uses potable water in substantial volumes. Managing the scaling effects and biological pollution of such water is a significant challenge. In locations where both humid and dry conditions occur, evaporative chillers are less effective. Standard liquid desiccant solutions do not operate well under those conditions. We will disclose how water addition can be controlled while simplifying liquid desiccant systems, making them competitive in both dry and humid conditions. We will also disclose that using vapor transfer modules in liquid desiccant systems reduces water consumption significantly over the life time of an installation when compared to traditional evaporative coolers, a critical consideration in many climate zones.
Many buildings have to deal with a variety of conditions from very hot and dry to relatively cool and humid including high DP/high relative humidity (RH) and high DB/low DP design points. We will disclose how liquid desiccant systems can be controlled to handle these conditions effectively. This includes the use of a combined liquid desiccant system with direct evaporation of the air supplied to the regenerator and indirect evaporative cooling of supply air after dehumidification. Both significantly improve system performance in dry and hot climates. They are identical in their effect on the system to direct dilution of the liquid desiccant by adding demineralized water to the liquid desiccant tank or using membrane modules to transfer water from a feedstream to the highly concentrated liquid desiccant.
Since the concentration in the liquid desiccant system drives the RH of both the supply conditions in the conditioner as well as the output conditions of the regenerator, liquid desiccant systems operate as “constant RH machines.” This allows for different control strategies, including controls based RH and WB targets and measurements rather than DB and DP. These can be simpler and more effective than traditional control in independently controlling humidity and temperature.
Measuring the concentration can be done in a variety of ways. We will disclose how this can be used for different control strategies, using a combination of RH of the regenerator exhaust and the conditioner supply, tank levels, electrical resistance, defraction, specific weight of the desiccant, and measuring concentration based on viscosity and temperature.
Desiccant dilution through vapor transfer or forward osmosis can be done in a variety of ways.
Controlling for crystallization uses empirical data on crystallization points at different temperatures and humidity levels. Preventing crystallization which stop the system's ability to manage humidity is critical. And since crystallization only occurs at conditions that are too dry for comfort, avoiding those conditions tend to improve supply conditions that most benefit building occupants.
Energy recovery is a major factor in managing air quality and the efficiency of air conditioning systems. We will disclose how this can impact control strategies.
Concentrated liquid desiccants are a very efficient form of energy storage with more KwH per lb. than ice. We will disclose how control strategies can make optimal use of regeneration capacity due to waste heat, solar or other heat resources.
Frost control is a critical consideration in any heatpump system. Liquid desiccant systems can avoid them by proper sizing of system components, improving overall system efficiency. Frost prevention strategies will be disclosed
Liquid desiccant systems have distinct modes depending on the relationship between input conditions and target conditions. We will discuss the main modes, their impact on bandwidth/adaptive systems as well as predictive systems. Including transition between modes without flip flopping.
SUMMARYIn accordance with one or more embodiments, a method is disclosed of operating a liquid desiccant air-conditioning system to maintain target temperature and humidity level in a space. The liquid desiccant air conditioning system comprises: (a) a conditioner for treating a first air stream flowing therethrough and provided to the space as a supplied air stream, said conditioner using a heat transfer fluid and a liquid desiccant to treat the first air stream; (b) a device for measuring temperature and a device for measuring humidity in the supplied air stream; (c) a regenerator connected to the conditioner such that the liquid desiccant can be circulated between the regenerator and the conditioner, the regenerator causing the liquid desiccant to desorb water vapor to a second air stream or to absorb water vapor from the second air stream depending on a selected mode of operation of the system; (d) a refrigerant system; (e) a first refrigerant-to-heat transfer fluid heat exchanger connected to the conditioner and the refrigerant system for exchanging heat between the refrigerant heated or cooled by the refrigerant system and the heat transfer fluid used in the conditioner; (f) a second refrigerant-to-heat transfer fluid heat exchanger connected to the regenerator and the refrigerant system for exchanging heat between the refrigerant heated or cooled by the refrigerant system and the heat transfer fluid used in the regenerator; and (g) a system controller for controlling operation of the system. The method comprises the steps of:
- (i) measuring the temperature and humidity level in the supplied air stream;
- (ii) comparing the temperature measured in (a) to a target temperature to determining a temperature error, and comparing the humidity level measured in (a) to a target humidity level to determine a humidity error;
- (iii) comparing the humidity error and the temperature error on a common scale to determine the greater error;
- (iv) using the greater error to drive the system controller to control operation of the system to reduce the greater error;
- (v) repeat (i) through (iv) a plurality of times.
The liquid desiccant is collected at the bottom of the wavy conditioner plates at 111 and is transported through a heat exchanger 113, to the top of the regenerator 102, and to point 115 where the liquid desiccant is distributed across the wavy plates of the regenerator. Return air, or optionally outside air, at 105 is blown across the regenerator plate and water vapor is transported from the liquid desiccant into the leaving air stream at 106. An optional heat source 108 provides the driving force for the regeneration. The hot transfer fluid at 110 from the heat source can be put inside the wavy plates of the regenerator similar to the cold heat transfer fluid on the conditioner. Again, the liquid desiccant is collected at the bottom of the wavy plates 102 without the need for either a collection pan or bath, so the regenerator the air flow can be horizontal or vertical. An optional heat pump 116 can be used to provide cooling and heating of the liquid desiccant. It is also possible to connect a heat pump between the cold source 107 and the hot source 108, which is pumping heat from the cooling fluids rather than the desiccant.
A valve system 441, 442, 443 and 444 is shown in the refrigerant circuit. For those skilled in the art it will be clear that a similar valve system in the heat transfer fluid circuit can be used to control the air cooled coils
The following fundamental approaches can be used to manage humidity and temperature independently:
1. Total cooling or heating capacity defined as the change in enthalpy of the system is driven by the capacity of the compressor system 416.
2. The liquid desiccant panels will use the available capacity to generate air with an RH that is significantly lower than the outside air used for regeneration. Fluid flows 418 and 440 for heat transfer fluids and 404 and 415 for liquid desiccant through the panels of 417 and 423 (air/heat transfer fluid/desiccant) can significantly adjust the ratio of latent versus sensible cooling.
- 3. Adding a sensible coil 429 to the condenser side of the compressor in parallel or in series to the regenerator 423 enables additional sensible cooling.
- 4. Adding a sensible coil 433 to the evaporator side 427 of the compressor with outside air or air from the regenerator 423 provides additional cooling power for deep dehumidification while maintaining or increasing the air temperature.
- 5. Adding a sensible coil 434 before the conditioner on the evaporator side of the compressor and after the conditioner on the airside maximizes sensible capacity.
- 6. Desiccant dilution 436 allows more sensible cooling. It can be used for net humidification of supply air and makes it possible to control humidity in a limited bandwidth.
- 7. Preconditioning the air 405 with direct evaporation has an identical effect to desiccant dilution, but uses existing components.
- 8. Exhaust air 431 can be used on the conditioner 427 to reduce the overall load, either sensible (Plate heat exchanger (HX)) 432 or sensible and latent (full enthalpy HX including wheels, plates, and LD plates).
- 9. Exhaust air 431 can be used at the regenerator 423 to provide deeper dehumidification.
The ability of the system in
For greater flexibility in managing temperature and humidity independently, additional components can be added to the system (
The main challenge for the system shown in
For a system that needs to be able to deal with a very broad range of conditions a simplified refrigerant circuit would be preferred with more of the adjustment in the system configuration being done on the heat transfer fluid side. That does involve an efficiency loss driven by the efficiency of the LCE 714 and the LCC 720. But it eliminates the need for multiple refrigerant switches, multiple expansion valves and their controls and for additional design to balance refrigerant and oil with receivers and accumulators.
Managing the various coils on the refrigerant side of the system becomes more difficult as the system gets more complex. A multi zone system is shown in
The heat pump system as shown in
Alternatively the air coils 922 and 671 can be connected indirectly to the compressor via the first and second refrigerant to heat transfer fluid coils 914 and 920. This significantly simplifies the refrigerant circuit but requires additional valves in the heat transfer fluid system.
For example, the task of controlling the system becomes complex if target conditions and loads of the conditioned spaces differ significantly. Some of the potential issues include:
Spaces with different input conditions of outside and return air to the conditioner. Some spaces may have exhaust air for regeneration others may not. Also the temperature and humidity of return air from the spaces may differ and of course the proportion of outside air required for the space, which often depends on occupancy and potential requirements for over pressurization.
Different loads in the space including high humidity loads from plants, pools, kitchens and people and high sensible loads from outside walls in older buildings, lights, equipment etc.
Different targets, e.g., in stores high humidity is desirable in green/veggie sections and low humidity is desirable in refrigerant sections. As a result the required load per cfm and the required concentration of liquid desiccant for matching user requirements could differ significantly.
Liquid desiccant control systems need to be able to address this by adjusting the water temperature and the liquid desiccant concentration supplied to a specific space. This may require a more complex tank system that allows the regenerator to adjust the concentration of the liquid desiccant by using different airflows and a mix of outside and exhaust air. Another option for creating multiple concentrations of liquid desiccant is to vary the temperatures and flows rates of heat transfer fluids supplied to the regenerator and the air cooled coils discussed above.
Referring to
Direct dilution ensures that the RH level of supply air 1106 cannot fall below a minimum level RHmin which can be calculated from the LD concentration LD % by a formula RH min=(100%−V×LD %)+effectiveness factor), where V is a factor driven by the vapor pressure of the liquid desiccant For LiCl V is about 2. The effectiveness factor is driven by the sensible and latent effectiveness of the panel and tends to be between 5-10% on average for cooling LD at a concentration of 25% will result in conditioned air at an RH of 55-60%. Minimum RH levels are critical for a wide range of applications. Optimal living and working conditions tend to have an RH between 40 to 70%. Maintaining a concentration of 20-30% ensures that those humidity conditions are always met. These results are based on extensive modelling of liquid desiccant systems over a broad range of conditions as well as tests of liquid desiccant panels.
An alternative desiccant dilution method is shown in 1100. Placing a direct evaporative pad or cooler in the incoming air stream of the regenerator or the conditioner also dilutes the liquid desiccant. A nozzle that creates a fine mist in the incoming air stream that quickly vaporizes has the same effect. During very dry conditions outside air conditions 1101 a conditioner 1103 using fan 1102 can supply cool air 1106 with a humidity at or just below a target DP. In this situation the conditioner 1103 can actually desorbs humidity from the liquid desiccant, increasing rather than decreasing the concentration and partially cooling the air. Evaporator coil 1100 in airstream 1146 humidifies and cools to a high DP. And a low temperature, The Regenerator 1148 will therefore absorb rather than desorb water vapor, diluting the liquid desiccant. In such extreme conditions the conditioner 1199 has a significantly lower load from evaporator 1114 which only has to provide the remaining sensible cooling to achieve target conditions. Therefore the heat load from condenser 1144 is low which results in a cool regenerator 1148 that absorbs water vapor, dehumidifying air 1146 to 1149 thus diluting the concentrated liquid desiccant from heat exchanger 1156 and pump 1153. Positioning tan evaporator pad 1100 in airstream 1101 the same effect, but exposes the airstream to the conditioned space 1106 to the evaporator pad, while regenerator air 1149 is exhausted. The overall effect of humidifying the regenerator air has the same effect as directly diluting the liquid desiccant. Instead of adding water to the desiccant through a vapor module, the water is added indirectly via the air. The advantage of this approach is that evaporative pads are cheap and well understood. The water management including managing water quality and mineral content with appropriate bleed streams will differ depending on location and water conditions. Suppliers of evaporative coolers are familiar with the water management issues. The cost of pads is currently lower than that of vapor transition modules. From a control perspective these two mechanisms require similar solutions.
Controlling water addition in liquid desiccant systems can be done in a number of ways described below: by increasing the feed stream either cyclically or through a variable speed pump, maintaining direct tank level control with a level sensor, adjusting flow rates of feed water to the evaporator etc.
Often solid desiccant wheels are used to recover energy from an exhaust air stream. The same can be done with a “passive” liquid desiccant systems.
A second set of panels does not use a compressor system or an external source of heat or cold. Instead it preconditions the incoming air 706, with water 801 and desiccant 717 which is pumped with 716 from tank 715. Panels 704 uses dry and cool exhaust air 102 to regenerate the diluted desiccant with the dry exhaust air 102, Water 802 has been warmed up by the absorption in 703 and is cooled by the low temperature of exhaust air 102. The desiccant regenerated in 903 uses the standard components including a liquid desiccant tank 712, a heat exchanger 718, and pumps 713 and 901. The tank allows for different concentrations of liquid desiccant as the exhaust air volume and outside air temperature and humidity change.
The liquid desiccant ERV in
The prior art shows the following fundamental tools to manage humidity and temperature independently in the liquid desiccant systems described above.
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- 1. Total cooling or heating is driven by the capacity of the compressor system.
- 2. The liquid desiccant panels will use the available cooling capacity to condition a combination of outside air, return air or air pre-conditioned by an energy recovery device or an evaporator unit. Higher airflows and/or higher heat transfer fluid flows through the regenerator panels can increase the ratio of sensible versus total cooling or the sensible heat ratio (SHR). Low liquid desiccant flows can further increase the SHR.
- 3. Adding an air cooled coil to the condenser side of the compressor in parallel or in series with the regenerator on the airside enables additional sensible cooling.
- 4. Adding an air cooled coil to the evaporator side of the compressor that processes outside air or air from the regenerator provides additional cooling power for deep dehumidification in the conditioner increases latent cooling, until latent cooling is larger than the total cooling capacity resulting in heating of the air If the air cooled coil provides all the load to the regenerator, the conditioner will dehumidify adiabatically, resulting in a negative SHR.
- 5. Adding a sensible coil before the conditioner on the evaporator side of the compressor and after the conditioner on the airside maximizes sensible capacity.
- 6. Desiccant dilution allows more sensible cooling and less dehumidification or even net humidification of supply air Significantly increase the relative humidity of the supply air, while reducing the temperature resulting in an SHR>1
- 7. Maintaining a minimum level of liquid desiccant with demineralized water maintains a minimum RH level, e.g. 30% with LD of about 35% concentration. Supply conditions can exceed this minimum RH level, based on input and ambient air conditions and fluid flows through the coils.
- 8. Preconditioning the air with direct evaporation has an identical effect to desiccant dilution, but uses existing components.
- 9. Exhaust air can be used on the conditioner to reduce the overall load, either sensible (Plate HX) or sensible and latent (full enthalpy HX incl. wheels, plates, and LD plates).
- 10. Exhaust air with a lower RH than ambient conditions can be used at the regenerator to provide more efficient dehumidification, since less condenser heat is required to maintain a high concentration of liquid desiccant.
Air-cooled coils and liquid desiccant heat exchangers can be connected either directly via the heat transfer fluid system or indirectly via the liquid cooled refrigerant heat exchangers when the air cooled coils condition air directly with refrigerant.
The above focusses on the use of fluid flow rates, sensible coils, desiccant dilution and exhaust air for independent management of latent and sensible cooling. It shall be clear to those skilled in the art that same options can be used for independent control of temperature and humidity with the refrigerant system in heating mode.
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- a. Cooling and deep dehumidification is a typical requirement for many air conditioning systems in hot and humid climates (4).
- b. Dehumidification with deep sensible cooling (3) requires maintaining or increasing the relative humidity typically during the heat of the day.
- c. Warming the air while dehumidifying (zone 5, 6) is a requirement with very humid but cool air typically in the early morning and in spring or fall in moderate zones.
- d. Humidification during cooling (1, 2) can be a benefit during cooling of very dry air, e.g. desert air at a DP below 40 F with a temperature of over 35 C. Typically this is a requirement in areas where liquid desiccant systems are essential for monsoon period (1604) but where part of the year or even day can be very hot and dry.
- e. In heating mode the main distinction is between zone 7 where high humidity can lead to frost forming in heat pump mode and to over humidification of the space and zone 8 where humidity is too low and additional humidification is required.
- f. Cool and dry air needs no conditioning and the system can operate in economy mode. Other than traditional systems, liquid desiccant solutions do have the option to humidify the air while maintaining dry bulb conditions. In essence that is a heating operation since the enthalpy of the air increases.
Economizer mode 1699b is a set of conditions where further processing will not occur and the air is supplied to the space as is.
In DOAS applications, the economizer zone 1699b can include zones 1 and 2 if the liquid desiccant unit is used as a highly efficient dehumidifier with an SHR that optimizes compressor performance. This maximizes overall system savings if highly efficient sensible-only systems are available for further cooling of the air, including but not limited to geothermal cooling and indirect evaporative cooling.
Dehumidification refers to a reduction in absolute humidity and/or relative RH. While humidification refer to an increase in absolute humidity.
Humidity control is driven by the temperature of the water 1744 and the air as 1746 as well as the humidity of the air 1746 which is supplied by fan 1747 to regenerator 1748. Pump 1740 rotates heat transfer fluid 1744 through refrigerant to water heat exchanger HX2 (1771) to regenerator 1748. Lower flows of water 1744 or lower flows of air 1746 result in a warmer regenerator and thus in more concentrated liquid desiccant which will result in dryer supply air at 1706. Air cooled coil HX3 1743 cools the hot and humid air from regenerator 1748, providing a load to the compressor 1799 to maintain a high temperature at the condenser 1771 in order to concentrate the liquid desiccant 1745. This is used during times where outside air 1701 is humid requiring dehumidification but cool, requiring sensible heating.
Heat dump HX4 (1770) runs in series or in parallel with liquid condenser coil 1771, reducing the heat available for regeneration and thus enabling the conditioner to cool 1708 more deeply without over-drying it. HX4 can be directly connected to the condenser 9 or via the refrigerant to water HX2 1771 (link 8)
Water addition option 5 either with the membrane module 1757 or in a desiccant tank 171710 is critical in zones 1 and 2 of
For those skilled in the art it will be clear that the solutions shown in
The target conditions are those supplied by the system to a space, which enable the space to maintain comfortable conditions. Most air conditioners supply air 1903 at a cooler and dryer condition than the comfort conditions 1902 in the space. This compensates for the sensible and latent loads due to occupancy, heat infiltration and the load of outside air. As described in ASHRAE's DOAS design guide, direct outside air systems can either supply room neutral conditions (1902) or ensure that all latent needs of the space are met (1904), plus at least some of the sensible cooling compared to input air condition 1901, with the remainder 1905 being cooled by sensible heat only coils, cold beam system etc.
Target condition 1906 is in a heating mode with sensible and humidity loads, i.e. warmer than comfort conditions.
Comfort zone conditions will be realized in the conditioned space after cooling and heating loads have been added at a given circulation rate.
The fan only or economizer mode is used during conditions at which no active air processing is required and the unit operates in economizer mode.
Economizer modes are a requirement for DOAS systems. ASHRAE recommends that the DOAS unit focusses on meeting the latent load requirements by bringing the air to a required DP condition. In general that condition will be lower than the target humidity, since the building will have latent loads, which need to be compensated for. The economizer mode 1910 of a DX system is limited to outside air that is already at target 1912 DP and need no further dehumidification. Otherwise the DX system needs to overcool and then reheat the air. A solid desiccant system will dry the air at temperatures significantly higher than comfort levels, leaving the full load to be carried by the DX system that recirculate the air.
Liquid desiccant air conditioning systems dehumidify and cool simultaneously. With sufficient water addition/evaporative cooling capacity they can reach any supply condition from any input condition 1911. In that case the economizer mode is limited to the comfort zone conditions. Without water addition LDAC systems maintain a maximum DP.
Typical recirculation systems have an economizer zone with a maximum but no minimum humidity level. This is a problem, since low humidity levels can be harmful for health reasons, building quality and some kind of equipment. LDAC units can be combined with water addition or evaporation to maintain both minimum and maximum humidity levels.
We will propose how such conditions can be managed simultaneously using adaptive control systems without stability issues caused by conflicts between proportional—integral—derivative controller (PID) controller loops.
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- a. The humidity absorbed at the conditioner 2305a needs to be identical to the humidity evaporated at the regenerator 2305b
- b. The RH at the conditioner and the RH at the regenerator are both in balance with LiCl at a concentration that is nearly the same. Typically when the liquid desiccant has a concentration of 25% going into the conditioner it will come out at a concentration between 23 and 24% and will return to 25% at the regenerator. As a result the conditioners RH is likely to be 2 to 4% higher than the RH at the regen for the vapor pressures in the air to be equal to the partial vapor pressure at the surface of the liquid desiccant. The RH of the system is 1-2× concentration ensuring the RH of the conditioner and regenerator are closely correlated within 5%.
Such a system will always supply air at a concentration what below that of the input condition, given that the energy available at the regenerator is always larger than the cooling power at the conditioner. Increasing the regenerator airflow will reduce the supply temperature at 2306b, which increases the RH of 2306B compared to 2304b. This will reduce the concentration of the liquid desiccant. Since the total cooling power remains the same, the WB condition will not change, but supply conditions will shift to a cooler, more humid condition 2306a. Increasing compressor power without changing the airflows will lead to a supply condition at a lower WB condition, but also to a lower RH at the regenerator and thus to deeper dehumidification at 2307a.
Ongoing discussion about the standard recognizes that in buildings with low latent loads and high sensible loads, e.g., from lights, supplying outside air at temperatures below comfort conditions could be justified. When selecting dehumidifiers for hot and humid climates the MRE may be more important than the ISMRE.
Still cooling outside air to 2359 and 2379 will lead to significant condensation on the coil while providing the additional load needed to reconcentrate desiccant for 920 D and C. 920B has been shown to balance to the 2310 supply condition without additional airflow over the sensible coil. 920A can match the 70/55 2310 condition by using the coil as a heat dump (2309) increasing the RH from 2310 in
Using evaporators 2403 prior to both the conditioner and the regenerator (2403) will provide both with input conditions 2413 at an RH of 75% or higher. The conditioner 2404 and the regenerator 2405 will be able to achieve the same supply and exhaust conditions 2410 and 2411 assuming a similar concentration of about 25% LiCl.
While traditional control sequences can also be used for such a system using bands of DB and DP conditions, the liquid desiccant also allows for an alternative approach show in
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- Condenser linked sensible coil or heat dump to reduce desiccant concentration and increase RH
- Evaporator linked sensible coil or advanced dehumidification coil to increase desiccant concentration and reduce RH
- Water addition or evaporative cooling pads to directly dilute the liquid desiccant and increase RH
- Increase regenerator airflow to cool the condenser at a lower temperature which reduces the concentration and increases RH
- Reduce water flow in the regenerator to increase the heat transfer fluid temperature and the condenser temperature which increases concentration and reduces RH
- Reducing heat transfer fluid flows at the conditioner to cool the air adiabatically and increase the DB temperature and reduce the sensible heat ratio
For those skilled in the art it will be clear that these examples are not limitative. Other options have been discussed including a post cooling coil linked to the evaporator to shift increase SHR.
Heating can be done similarly by allowing independent control of the compressor for overall heating effort and the humidity controls and frost prevention controls using the sensible coils, fluid flows and water addition/desiccant dilution.
In addition to using tank levels, desiccant concentration can be measured in a variety of ways. A simple measure is the RH out of the regenerator or conditioner. For LiCl the percentage RH is approximately 1-2*concentration LD. Direct sensors are another options however the cost reliability and accuracy of these sensors needs to be further demonstrated. Multiple sensors in a system allow for more accurate supply conditions, more effective predictive controls with faster response times and system diagnosis e.g. detection of small leaks based on readings of different sensors.
Liquid desiccant system with RH sensors in the regenerator exhaust, where the RH is used to calculate the LD desiccant concentration using a formula based on a combination of basic physics (equal vapor pressures at equal RH at any temperature) and system know how (Latent effectiveness of the panels). 1−X1(ld)*C−LD+Delta(system)=RH, where X1(LD) is determined by the type of liquid desiccant used (ca 2-2.1 for LiCl) and Delta is driven by the system dimension (1-5%).
A Liquid desiccant system with a ventricle (narrowing of the pipe) can measure concentration based on pressure drop over the ventricle at a given flow rate.
Specific weight can be used by using a floater with known specific weight in the LD, In that case the depth of the floaters determines the concentration.
Diffraction uses the changing optical properties of the LD to measure the diffraction of a known frequency laser beam to measure LD concentration.
Electrical resistance uses changing electrical properties of the LD to determine concentration. However for LiCl resistance plateaus in the critical 20-30% range which requires this measure to be used in combination with other system properties, e.g. RH out.
Alternative adaptive control algorithms that focus on the specific properties of the single step liquid desiccant cooling and dehumidification system include therefore the enthalpy/RH control, the tank level control. A potential problem with a single step adaptive control system is that controlling multiple variables at the same time can lead to conflicting PIC loops. This can lead to oscillations of the system control. A way to get to a single PID loop is to first determine whether temperature or humidity level is further from target. Most system measure Temperature and RH. The controller can covert that to enthalpy and absolute humidity conditions, i.e., WB and DP or RH. The system PID loop can approach the target conditions for temperature and humidity by determining which of the two variables is furthest of target. Whether that is the delta of actual versus target DB or WB or the target versus actual DP or RH.
A similar structure can be used for the adaptive control method shown in
Here the crystallization control 3631 is shown to be driven by the target DP and measured RH values for conditioner and regenerator.
Such an approach can use any combination of WB/RH/DB and DP to control the system. However WB/RH, and DB/RH and DP controls are particularly important. The former can be used in a bandwidth controller. DB/RH is directly based on typical user settings. DP control is important when the Liquid desiccant unit is used as a dehumidifier with separate sensible cooling capacity.
The Actual DB condition in the supply airstream 3805 can be compared to the target 3802 to calculate DB error 3808. The controller 3809 identifies the larger of the two errors and uses that to drive the Compressor setting 3810. The regenerator fan speed 3810 is adjusted based on the RH error to approach the DB condition as much as possible than the Actual RH has to be used in combination with the actual DB to calculate the actual DP and thus the DP error. The largest error is again the driver of the compressor through the PID. While the RH error can adjust the regenerator fan to get closer to the target humidity condition. If the DB target is translated in an RH at the target DP, than DP/RH can be used, with RH driven by fluid controls for the various coils and the regenerator and the DP kept on target with the compressor speed.
The conditioner fan in can be variable or fixed but is set independently of the other variables within an allowed bandwidth. Especially in outside air unit, conditioner airflow 3818 should not be used to control conditions. This is of course different in recirculation units where airflow is one of the controls of the PID controller.
Input conditions to the conditioner and regenerator can be compared to target 3820 and can be used to accelerate the adaptive controls 3821. This can also be used as the main control esp. in outside air systems. The controller also has to automatically adjust valve settings 3821. In combination with ambient condition forecast or supply requirement forecast the controller can be optimized to anticipate future demand and capacity (Ambient air temperature and humidity) for dehumidification.
Controls for liquid desiccant systems are focused on humidity management. Using temperature as the primary driver can be necessary where the unit is the only option. However in that case additional sensible cooling capacity needs to be added. A coil in the supply air from the conditioner to the space heated by the condenser (hot gas reheat) is an effective alternative to the condenser coil and has comparable efficiency. Both the advanced dehumidification coil and the hot gas reheat coil operate at low compressor lifts during cool and humid conditions. Both provide additional load to the compressor respectively for concentrating the liquid desiccant and for overcooling air and then reheating it. The choice will be driven by the cpst of the more complex refrigerant systems with another hot gas coil and by the benefits that the advanced dehumidification coils have in heating mode, in particular the frost free heating avoiding or significantly reducing defrost cycles which is made possible by the additional coil.
Such continuous learning and improvement can be integrated in the system. Alternatively it requires monitoring of field systems to optimize controls based on actual usage. It also requires an ability to upgrade software controls remotely. This is a critical requirement for liquid desiccant systems. Users are not familiar with the capabilities of these systems and are used to the limitations of the traditional two step control systems. Remote monitoring and control will accelerate learning from the user community and improve performance and acceptance of the systems. Wireless connections enable high reliability usage of the system.
Remote monitoring of the input and output conditions as well as the actual settings of the system is also a primary driver of preventive maintenance where deviations from historical performance indicate potential problems with system components.
Increasingly manufacturers use their pool of customers to learn what gives the best performance. An adaptive liquid desiccant system will combine direct feedback from field units with app based feedback from users. This can use a combination of smart algorithms with “wisdom of the crowd” based selection among those algorithms. For this to be useful a variety of settings for the units is needed. For example, humidity could be more of a problem when people are active or when they are trying to sleep than when they sit and relax. That could influence how much humidity levels fluctuate and how fast the system needs to respond. There are many crowd based systems that allow gathering that kind of info. Combining these systems with info on liquid desiccant air-conditioning solutions with a focus on humidity will lead to rapid acceptance of such systems.
Predictive controls shown in 38 C can take different forms.
Predictive controls based on the existing detailed system models which can use input conditions to set compressor and humidity controls to quickly move to target
Learning models that actively perturbs (within limits) unit settings to measure effect on efficiency and learns the combinations of operating parameters that work best to minimize energy usage.
Predictive models are particularly important when there are large cycles in humidity during the day, or predictable swings in requirements (weekends) or time of year. For example a model can how dehumidification requirements and sensible loads will vary over the course of a day. Early in the day, outside air conditions will be humid and cool, requiring highly concentrated desiccant. During the middle of the day large sensible loads produce the heat required to deeply concentrate the liquid desiccant. Using a sufficient volume of liquid desiccant allows storage of concentrated liquid desiccant that can be used during cool and humid nights. Similarly at the end of the day, where cooling and dehumidification requirements are likely to remain large until well after sunset, loads can be reduced by using solar heat or solar power to store highly concentrated desiccant to reduce demand in the early evening where the “duck shaped” network power demand curves could benefit from shifting some of the load from early evening to late afternoon.
Predictive models can use a combination historical data and last minute data, similar to the pricing models of airlines. Airlines set prices based on day of week and date, but then adjust based on actual buying behavior. A liquid desiccant predictive model can similarly use day of year/and hourly data to predict settings and then adjust based on last 24 hours and current data. For example this could allow a system to go into special dehumidification mode in September if temperatures the day before were 70 F DP 60 F+, but stay in standard cooling mode in July when that probably represents a rainy day, below seasonal averages.
Bandwidth control systems will show fluctuations between the upper and lower boundaries of the bandwidth. However given that the building changes all the air only once every 10-30 minutes fast response is often not required. Bandwidth control is simpler and can work effectively with a liquid desiccant system by using a combination of WB and RH bandwidth and measurements to drive the controls. Any input settings (DB/DP/RH) can be used to calculate the WB and RH bandwidth.
Referring to
Liquid desiccant systems do need to control for crystallization.
Avoiding crystallization requires that nowhere in the panels the temperatures and humidities shown in
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- Use of fixed flow pumps to ensure that flow rates at conditioner and regenerator are equalized.
- Use of pressure controls through valves or air references to maintain a higher pressure for more viscose desiccants.
- Accept differences in flow rates on the conditioner and regenerator, the tanks or valves can be used to balance the regenerator and conditioner.
Having thus described several illustrative embodiments, it is to be appreciated that various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to form a part of this disclosure, and are intended to be within the spirit and scope of this disclosure. While some examples presented herein involve specific combinations of functions or structural elements, it should be understood that those functions and elements may be combined in other ways according to the present disclosure to accomplish the same or different objectives. In particular, acts, elements, and features discussed in connection with one embodiment are not intended to be excluded from similar or other roles in other embodiments. Additionally, elements and components described herein may be further divided into additional components or joined together to form fewer components for performing the same functions. Accordingly, the foregoing description and attached drawings are by way of example only, and are not intended to be limiting.
Claims
1. A method of operating a liquid desiccant air-conditioning system to maintain target temperature and humidity level in a space, the liquid desiccant air conditioning system comprising:
- a conditioner for treating a first air stream flowing therethrough and provided to the space as a supplied air stream, said conditioner using a heat transfer fluid and a liquid desiccant to treat the first air stream;
- a device for measuring temperature and a device for measuring humidity in the supplied air stream;
- a regenerator connected to the conditioner such that the liquid desiccant can be circulated between the regenerator and the conditioner, the regenerator causing the liquid desiccant to desorb water vapor to a second air stream or to absorb water vapor from the second air stream depending on a selected mode of operation of the system;
- a refrigerant system;
- a first refrigerant-to-heat transfer fluid heat exchanger connected to the conditioner and the refrigerant system for exchanging heat between the refrigerant heated or cooled by the refrigerant system and the heat transfer fluid used in the conditioner;
- a second refrigerant-to-heat transfer fluid heat exchanger connected to the regenerator and the refrigerant system for exchanging heat between the refrigerant heated or cooled by the refrigerant system and the heat transfer fluid used in the regenerator; and
- a system controller for controlling operation of the system;
- wherein the method comprising the steps of:
- (a) measuring the temperature and humidity level in the supplied air stream;
- (b) comparing the temperature measured in (a) to a target temperature to determining a temperature error, and comparing the humidity level measured in (a) to a target humidity level to determine a humidity error;
- (c) comparing the humidity error and the temperature error on a common scale to determine the greater error;
- (d) using the greater error to drive the system controller to control operation of the system to reduce the greater error;
- (e) repeat (a) through (d) a plurality of times.
2. The method of claim 1, wherein the temperature measured in (a) is a dry bulb temperature and the humidity level measured in (a) is a relative humidity level, and wherein the target temperature is a dry bulb temperature and the humidity target is a dew point target;
- and the step (c) is based on errors in dry bulb and dew point.
3. The method of claim 2, wherein when the system operates as a dehumidifier, the system controller is operated to prevent the dew point based on measurements from being lower than the target dew point.
4. The method of claim 2, wherein the system controller controls the setting of a compressor in the refrigerant system to control the cooling capacity of the system, and
- the system controller controls the liquid desiccant and heat transfer fluid flow rates in the regenerator, wherein higher liquid desiccant and heat transfer fluid flow rates in the regenerator increase the sensible cooling rate of the supply air stream.
5. The method of claim 4, wherein the liquid desiccant air conditioning system further comprises a liquid desiccant dilution device, and the method further comprises controlling the sensible cooling rate of the supply air stream by diluting the liquid desiccant using the liquid desiccant dilution device.
6. The method of claim 5, wherein the liquid desiccant dilution device is used to maintain minimum level of liquid desiccant in a liquid desiccant tank of the liquid desiccant air conditioning system to maintain a minimum relative humidity level in the supply air stream.
7. The method of claim 4, wherein the liquid desiccant air conditioning system further comprises an air-cooled coil associated with the second refrigerant-to-heat transfer fluid heat exchanger, the method further comprises increasing the sensible heat ratio by increasing fluid flow rates through the air-cooled coil.
8. The method of claim 4, wherein the liquid desiccant air conditioning system further comprises an air-cooled coil associated with the first refrigerant-to-heat transfer fluid heat exchanger, the method further comprises reducing the sensible heat ratio by increasing fluid flow rates through the air-cooled coil.
9. The method of claim 1, wherein the temperature measured in (a) is a dry bulb temperature and the humidity level measured in (a) is a relative humidity level, and wherein the target temperature is a dry bulb temperature and the humidity target is a dew point target;
- and the step (c) is based on errors in wet bulb and relative humidity.
10. The method of claim 9, wherein the wet bulb error controls the setting of a compressor in the refrigerant system to control the cooling capacity of the system, and
- the relative humidity error controls the liquid desiccant and heat transfer fluid flow rates in the regenerator, wherein higher liquid desiccant and heat transfer fluid flow rates in the regenerator increase the sensible cooling rate of the supply air stream.
11. The method of claim 10, wherein the liquid desiccant air conditioning system further comprises a liquid desiccant dilution device, and the method further comprises controlling the sensible cooling rate of the supply air stream by diluting the liquid desiccant using the liquid desiccant dilution device.
12. The method of claim 11, wherein the liquid desiccant dilution device is used to maintain minimum level of liquid desiccant in a liquid desiccant tank of the liquid desiccant air conditioning system to maintain a minimum relative humidity level in the supply air stream.
13. The method of claim 9, wherein the liquid desiccant air conditioning system further comprises an air-cooled coil associated with the second refrigerant-to-heat transfer fluid heat exchanger, the method further comprises increasing the sensible heat ratio by increasing fluid flow rates through the air-cooled coil.
14. The method of claim 9, wherein the liquid desiccant air conditioning system further comprises an air-cooled coil associated with the first refrigerant-to-heat transfer fluid heat exchanger, the method further comprises reducing the sensible heat ratio by increasing fluid flow rates through the air-cooled coil.
Type: Application
Filed: Nov 1, 2018
Publication Date: May 23, 2019
Inventors: Mark D. Rosenblum (Woburn, MA), Scott N. Rowe (Dover, NH), Peter F. Vandermeulen (Newburyport, MA), Eric Kozubal (Superior, CO), Jason D. Woods (Boulder, CO), Peter Luttik (Beverly, MA)
Application Number: 16/178,305