Method and System for Improved-Efficiency Air-Conditioning

Abstract

The amount of supply air used by an HVAC system, and hence the amount of energy used for heating and cooling, while maintaining desirable air quality and composition, is reduced by removing unwanted gases, such as carbon dioxide, using scrubbers or other devices that separate these gases from the circulating air. Optionally, the air can be further improved with injection of concentrated oxygen. While in a normal HVAC system frequent replacement of the building air is performed, scrubbing of CO2 and other unwanted gases, with or without additional oxygen, would achieve the same goal, but with less frequent air replacement and therefore lower thermal load on the HVAC system.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a nonprovisional application of, and claims priority to and the benefit under 35 U.S.C. 119 of, provisional patent application 61/345,194, filed May 17, 2010, and provisional patent application 61/351,968, filed Jun. 7, 2010, each of which is hereby incorporated by reference.

BACKGROUND

Heating, Ventilation and Air-Conditioning (HVAC) are standard in virtually every modern building. Indeed, HVAC is often the largest part of the entire energy budget of most buildings, and this is particularly the case in extreme climates, both hot and cold. The goal of HVAC systems is to provide comfortable and healthy conditions for the building occupants, in terms of temperature, humidity, composition and cleanliness of the air.

Central HVAC systems typically include one or more central air handling unit and an air distribution system, where supply air is directed to the various parts of the building through a network of ducts, and return air flows from these spaces, through ducts or a plenum, back to the air handling unit. In the air handling unit, air is cooled or heated, as well as filtered and often dehumidified or humidified, as needed. Thus HVAC systems constantly circulate air through the building while continually adjusting is temperature and humidity to maintain comfortable conditions.

However, in order to maintain good air quality, not all the air is recirculated: some fraction of the circulating air is constantly exhausted outside the building—hence exhaust air—and is replaced by an intake of outside air also known as makeup air, to make up for the exhaust air. In other places this is also referred to as “fresh air” or ventilation. This replacement of the air is done because the occupants of the building and the equipment consume oxygen and emit carbon dioxide (CO₂) and a variety of other contaminants that would gradually compromise the quality and safety of the air. This replacement of the air maintains fresh air quality.

Oxygen represents about 21% of atmospheric air and that is normally the desired level of indoor air as well. On the other hand CO₂ is present only in very low levels in outside air, typically a few hundred ppm (parts per million). Once breathing produces elevated levels of CO₂ and some of the indoor oxygen is consumed, a fairly significant amount of outside air is used to bring their respective concentrations close to the desired level. Indeed, to fully restore oxygen and CO₂ concentration virtually all the air would need to be replaced.

The outside air represents an additional, and—depending on outside climate conditions—often a significant, thermal load on the air handling unit. In the case of a hot and humid climate, for example, the outside air injected into the HVAC system requires additional energy for cooling and dehumidifying the outside air, and can represent a significant fraction of the entire thermal load, hence energy usage, of the HVAC system.

The amount of exhaust air and outside air can adjusted to meet the air quality standards. A certain minimum amount is often set to maintain air quality, in terms of levels of oxygen, CO₂ and other contaminants. In the USA, the American Society of Heating, Refrigeration and Air-conditioning Engineers (ASHRAE) issues guidelines for the minimum amount of outside air ventilation recommended for a given space and number of occupants. However, the greater the rate of air replacement, the more energy is consumed by the HVAC system.

SUMMARY

The amount of supply air used by an HVAC system, and hence the amount of energy used for heating and cooling, while maintaining desirable air quality and composition, is reduced by removing unwanted gases, such as carbon dioxide (CO₂), using scrubbers or other devices that separate these gases from the circulating air. Optionally, the air can be further improved with injection of concentrated oxygen. While in a normal HVAC system frequent extensive replacement of the building air is performed, scrubbing of CO₂ and other unwanted gases and vapors, with or without additional oxygen, would achieve the same goal, but with much lower thermal load on the HVAC system, providing significant energy saving for the building and reducing demands on the entire electrical grid.

In one embodiment, the HVAC system also has an oxygen injection system that injects oxygen-enriched air into the circulated air.

In one embodiment, a control system for use with an HVAC system has a gas scrubbing system for removal of an unwanted substance gas from circulated air. The control system includes a sensor for determining an amount of the unwanted substance gas in the circulated air. controller modifies a rate of exhaust of circulating air and intake of outside air so as to adjust overall air replacement according to the measured amount of unwanted substance gas in the circulated air. The control system also can include an oxygen sensor for determining an amount of oxygen in circulated air, and wherein the controller modifies the rate of oxygen injection.

In another embodiment, the system is a modular system can be connected to an HVAC system that circulates air in an enclosed environment. The modular system comprises a module for scrubbing configured to reduce a level of an unwanted substance in the circulating air.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a conventional HVAC system

FIG. 2A illustrates an HVAC system incorporating CO₂ scrubbing and oxygen injection.

FIG. 2B illustrates another embodiment of the system of FIG. 2A.

FIG. 2C illustrates another embodiment of the system of FIG. 2A.

FIG. 3 shows the configuration of valves and lines allowing the scrubber to switch from adsorption mode to purge mode.

FIG. 4 illustrated the addition of an oxygen injection system into the system of FIG. 2A.

FIG. 5 is a diagram of the control flow for a controller for such an HVAC system.

DETAILED DESCRIPTION

FIG. 1 schematically describes a typical circulating central HVAC system. A central air handling unit has both heating and cooling elements, which modify the temperature of the circulating air as it flows and comes in contact with these elements. Fans or blowers force the flow of the conditioned supply air through ducts that distribute the conditioned air throughout the various parts of an occupied space (an enclosed environment). In this document, a building is used as an example of an enclosed environment may have different zones for which the rates of air flow are different. Return air flows back to the air handling unit, as indicated at 10, and can be filtered to remove particles, bacteria, and various fumes. However some of the return air is exhausted outside the building, through valves that control the amount of exhaust released. At the same time fresh outside air is pulled in to replace the exhaust air and maintain the correct overall volume and pressure of air in the building. Typically 10-15% of airflow is released as exhaust and replaced, but this number can vary widely. Indeed some settings like bathrooms and kitchens will exhaust and replace 100% of the air flow. The constant replacement of exhaust air with outside air helps maintain good air quality, and in particular replenish oxygen consumed by the building occupants and remove carbon dioxide and other compounds generated by the occupants or the equipment inside the enclosed environment.

The enclosed environment can be an office building, commercial building, residential building, house, school, factory, hospital, store, mall, indoor entertainment venue, storage facility, laboratory, vehicle, aircraft, ship, bus, theatre, enclosed arena, education facility, library or other enclosed structure which can be at times occupied by breathing things, such as humans or animals.

FIGS. 2A, 2B and 2C schematically show how to incorporate scrubbers in the HVAC system in order to allow reduction of exhaust air and outside air. The scrubber (CS) intercepts some of the flow of return air, allowing scrubbed air to continue to flow to the air handling unit and back into the building, but CO₂ and other compounds are captured or filtered. The scrubber can be implemented in many ways, as CO₂ scrubbing has been used for decades in industrial applications as well as in spacecraft and submarines.

In one embodiment the CO₂ scrubber utilizes a bed of adsorbent material, such as synthetic zeolite, placed in a container, canister or lining the inside of one or more tubes. Several zeolites have been shown to be effective adsorbents of CO₂, notably zeolite-13X. These are readily available from a variety of commercial sources, such as W.R. Grace SYLOBEAD® C-Grade 13X, Pingxiang XINTAO Chemical Packing Co., Ltd. In China, GHCL Ltd., in India, and many others. Indeed, zeolite beds have been developed to extract CO₂ from a gas stream for various industrial applications (Ventriglio et al, 1968; U.S. Pat. No. 3,619,130; Reyhing et al, 1971, U.S. Pat. No. 3,808,773; Collins, 1972, U.S. Pat. No. 3,751,878; Shermen et al, 1974, U.S. Pat. No. 3,885,927; Sirkar et al, 1979, U.S. Pat. No. 4,249,915; Grenier et al, 1991, U.S. Pat. No. 5,137,548). The same underlying technologies can readily be adopted for this invention, which in fact is more forgiving in terms of the allowed residual CO₂ in the outflow. In certain embodiments adding other adsorbents, including multiple zeolites, porous alumina (Slaugh et al, 1981, U.S. Pat. No. 4,433,981; Kumar et al, 1986, U.S. Pat. No. 4,711,645) or the long established activated charcoal (Allen, 1921, U.S. Pat. No. 1,522,480; Bechthold, 1927, 1,836,301) may further improve air quality or energy efficiency by removing other gases, volatile organic compounds and humidity or by allowing lower-temperature release of adsorbates. In some embodiments the combination of several different adsorbents in the same unit or as separate units may offer the best performance. As such captured gases accumulate in the scrubber, at some rate these need to be removed from the scrubber, in what is commonly called “regeneration”. These unwanted gases can be released to the atmosphere or otherwise collected and disposed of or sequestered. In one embodiment the release is achieved by a combination of heating and purging with air or other purge gas. Thus an adsorption-desorption cycle sometimes referred to as temperature swing adsorption. During regeneration the scrubber is isolated from the HVAC circulation by a set of valves, shown in FIG. 3, and in turn connected to the incoming and outgoing purging lines. During the adsorption cycle Valve 1 and Valve 2 are open, connecting the scrubber to the circulating air flow, while Valves 3 and 4 are closed. During regeneration Valves 1 and 2 are closed and Valves 3 and 4 are open, flowing purge gas thru the scrubber while isolating it from the air circulation system. If the scrubber regeneration interrupts the continual scrubbing process for an unacceptably long period of time, multiple scrubbers (not shown) may be used to avoid such interruption, so that when one scrubber is undergoing regeneration, another scrubber is engaged. However, short interruptions may not pose a problem, as long as the aggregate amount of CO₂ removed over periods of several hours is sufficient. Similar back up may be implemented for the oxygen concentrator.

The scrubber adsorbent bed design will include the appropriate choice of adsorbent material, its amount, its spatial distribution, the air flow pattern and its overall capacity to be compatible with the airflow design requirements. There are tradeoffs to consider in terms of system size and cost versus throughput, frequency of regeneration and energy requirements for regeneration. The amount of CO₂ that can be collected and released in each temperature swing adsorption cycle is dependent on the amount of active and accessible adsorbent material, as well as the temperature gap between the adsorption and purge cycle. Thus to achieve a certain rate of gas capture one use less material and operate with more frequent purge cycles. However there are natural kinetic rates for adsorption and desorption that depend on material and temperature that constrain the cycle time for a given amount of material. To minimize the energy required, i.e. the energy required to heat the purge gas, one would design a lower purge gas temperature, however that would reduce the amount desorbed per cycle. In an application that is primarily driven by energy savings, one can start with the temperature and volume of purge gas that can be produced by the excess heat of the HVAC system and use that to design the thermal range of the temperature swing cycle, and based on that and the kinetics of the adsorbent design the dimensions of the bed. It is anticipated that different embodiments will be implemented in different settings to address these tradeoffs.

Solid adsorbents like zeolite 13X offer a preferred embodiment but there are many other ways to remove CO₂ as well as other unwanted gases and vapors. In other embodiments CO₂ scrubbing is achieved by reactions with alkaline hydroxide bases. In another embodiment CO₂ scrubbing is achieved with amine gas solutions, such as monoethanolamine or other amines, that are well known in the art. Another embodiment scrubbing is achieved by a chemical cycle in which sodium carbonate combines with carbon dioxide and water to form sodium bicarbonate (Fuchs, 1967 U.S. Pat. No. 3,511,595). Yet other techniques for removal of CO₂ include selective membranes, for example, PRISM membranes from Air Products, Inc, or CYNARA membranes from Cameron International Corp. Since the scrubber is a separate module in this systems, as new scrubbing technologies emerge they can readily be replaced in such a system without having to change its other components.

The scrubber will have to be regenerated and many of the above techniques require heat for regeneration. Some of that heat can be obtained by harvesting waste heat produced by other systems nearby, including the compressor and the air handling unit of the HVAC system, as well as solar energy. This could further improve the overall economics of the system. In certain embodiments the purging of the adsorbent bed utilizes warm air from the cooling unit to purge the bed during regeneration. In some embodiments solar energy is collected on a rooftop unit and used to heat the purge gas. Solar heating and harvesting compressor heat and other wasted heat can be used in combination, to minimize the energy usage of the system as a whole. Independent or additional heating may be performed to achieve a particular purge gas temperature in which case a heating coil, a furnace or a gas burner can be incorporated to the system before the entry point of the purge gas.

FIG. 2A shows the scrubber (CS) intercepting all of the return air flow. In an alternative configuration, shown in FIG. 2B, only some of the return air is diverted to the scrubber while the rest bypasses the scrubber and flows directly to the air handling unit. It is not essential that all the air pass thru the scrubber, as long over time a sufficient fraction of the unwanted gases are captured. In another configuration, the scrubber is positioned downstream from the air handling unit, which has the advantage of colder air entering the scrubber and cooling it. Most scrubbers, and adsorbents in particular, perform better with lower temperatures. From an air quality standpoint, the any location of the scrubber can work, as long as there is over time adequate amount of contact between the circulating air and the scrubber somewhere along the flow path of the air before or after the air handling unit. The scrubber(s) could even be distributed in the occupied space.

The scrubber will collect CO₂ and potentially other substances that can be disposed of in various ways. They could be released to the atmosphere, or collected in containers for handling and disposing in another location, or flowed through pipelines to another location or facility, to be stored, processed or utilized. For example, CO₂ is beneficial for greenhouses and could be directed to such greenhouses by pipes or by containers. Alternatively these byproduct gases can be sequestered indefinitely simply to avoid releasing them into the atmosphere. However there will be a higher cost to such disposition of these gases and it will not necessarily be economically justifiable to do so.

FIG. 4 illustrates the addition of an oxygen concentrator (OC) to the system. In this embodiment, an oxygen concentrator takes its own outside air supply (OA2) and creates a flow of concentrated oxygen (O), which is directed through an additional intake valve in to the air handling unit, upstream from the heating/cooling elements. The oxygen concentrator disposes of nitrogen and potentially other by-products back to the atmosphere as indicated at N. The amount of oxygen added to the circulating air depends on flow rate and the oxygen concentration. The latter could be well over 90% as is the case in most commercially available concentrators, but even a lower concentration would achieve the desired results, with a slightly higher flow rate.

The oxygen concentrator can be implemented in many ways. In the preferred embodiment, the technique for oxygen concentration is Pressure Swing Adsorption (PSA) or Vacuum Swing Adsorption (VSA). This technique has been known since the 1960's, it is in widespread commercial use today, and is readily available from a variety of producers making many products with different sizes and output capacities, as stand-alone systems for providing concentrated oxygen directly from air. Example VSA oxygen generating systems include, but are not limited to, the PRISM VSA oxygen generation systems from Air Products Inc.; the OXYSWING product line from Innovative Gas Systems, Inc.; the ADSOSS line of oxygen generators from Linde; the VPSA oxygen generating system from Praxair Inc. These PSA/VSA systems utilize highly porous adsorptive solids, usually a synthetic zeolite bed, in one or more container, typically shaped as a cylindrical column, and use pumps and compressors to change the pressure of gases in these containers. The technique relies on differential adsorption of oxygen and nitrogen onto the adsorbent. Thus it takes an inflow of normal air (or other gas mixtures), and generates two separate outputs: oxygen concentrated air and oxygen depleted air. The advantage of PSA/VSA is that these systems can continually generate oxygen for extended periods without much maintenance.

Other ways to separate or concentrate oxygen exist. Cryogenic separation is an effective way for large volumes and high purity, where the different condensation/boiling temperatures of different gases are used to separate oxygen from air. Selective membranes and selective diffusion media have also been developed to separate oxygen from air. Concentrated oxygen can also be generated from electrolysis of water, where electrical current through water generates oxygen gas on one electrode and hydrogen gas at the other. While these are energy intensive processes, pure hydrogen or nitrogen created as by products and can be collected and utilized for other applications.

Even the presence of both the scrubber and the oxygen concentrator does not mean that exhaust air and outside air are necessarily eliminated altogether. In certain embodiments, exhaust air and outside air will be kept at a controlled level, lower than in a conventional HVAC system but a level that would still be warranted or desired in order to assure that there is no gradual deterioration in air quality despite the benefits of the oxygen concentrator and the scrubber.

We describe systems both with and without the oxygen concentrator. Indeed in some embodiments the oxygen concentrator is eliminated, and the use of a scrubber by itself imparts the majority of the benefits. At first glance this may not be obvious, since oxygen consumption and CO₂ emission go hand in hand and occur in almost identical molecular quantities, which implies that the drop in oxygen concentration would be commensurate to the rise in CO₂ levels, and the sum of the two almost constant. However, as long as makeup air is reduced but not eliminated altogether, even without a scrubber and an oxygen source, the oxygen and CO₂ levels will stabilize at certain asymptotical concentrations that together sum up to 21%, the same as that of outside air. The asymptotic level of oxygen, X, is given by

X=X₀−B_o/M

Where X₀ is the concentration of oxygen in outside-air, B_o is the net amount of oxygen consumed (in CFM, liters/second or any other units) by the occupants and M is the amount of outside air injected (in same units, CFM, liter/second, etc respectively). Similarly CO₂ level, Y, would be given by

Y=Y₀+B_c/M

where Y₀ is the concentration of CO₂ in outside-air and B_c is the net amount of CO₂ exhaled by the occupants. Looking at the above it is clear that as long as B_c≈B_o, at least approximately, then X+Y≈X₀+Y₀. However, adding a scrubber that extracts CO₂ at a rate of S_c (in same units, CFM, liter/second, etc. respectively) will result in

Y=Y₀+(B_c=S_c)/M

Analogously the impact of an oxygen generator injecting at a net rate of G_o (in same units, CFM, liter/second, etc., respectively) would be to change the asymptotic value of X to

X=X₀−(B_o−G_o)/M

For example, if outside air is at the normal 21% oxygen, and occupants consume 2 CFM net oxygen and exhale a similar amount of CO₂, makeup air is at 100 CFM, and no further scrubbing or oxygenation are in effect, then oxygen will gradually approach 19% while CO₂ approaches 2%. But whereas a 19% concentration of oxygen may be acceptable in some circumstances, a 2% concentration of CO₂ is clearly not. Thus adding a scrubber with S_c=2 CFM capacity alone could bring CO₂ levels down to normal. Oxygen will still be at approximately 19%, unless we inject supplemental oxygen, but even so air quality may be acceptable at this level even without an oxygen source, and would require less hardware and less operating costs, therefore might be a preferred embodiment for some buildings.

FIG. 5 shows how air quality is maintained through a feedback system. Sensors (Y) are distributed through the building space and detect levels of one or more target gases, such as CO₂ and/or oxygen but potentially also other gases. Sensors for CO₂ are commercially available, examples include the C7232 sensor from Honeywell Corp., TELAIRE sensors from General Electric. A central control system (CC) can be human operated, automated or computerized. The control system detects the signal for said sensors and, based on these and the various parameters and settings of the system, controls or modifies any of the following, in order to achieve targeted conditions: OC power (on/off), OC settings, OC valves, CS settings, CS regeneration trigger, outside air flow rate, exhaust air flow rate. The system can have fail safe measures to prevent unwanted elevation of oxygen, and the ability to shut down either or both oxygen concentrator and scrubber if needed and compensate by increasing outside air and exhaust air levels to those of a conventional HVAC.

The control system can permit the amount of scrubbing or injection of oxygen to be adjustable, whether directly or indirectly, whether electronically or manually. Adjustments can be achieved by changing the power or settings applied to the various compressors, pumps, motors, heaters, actuators or valves associated with the scrubbers and the oxygen concentrators. The adjustments to the amount of scrubbing or oxygen injection can be automatically done in response to a measurement of air quality or air composition in one or more locations. The adjustments to the amount of scrubbing or oxygen injection can also be automatically done based on building occupancy, time of day, day of the week, date, season or outside climate.

In one embodiment, the scrubber is set to run at a constant operating mode. The capacity and efficiency of the scrubber in that mode should be selected based on the occupied space and the amount of activity in the occupied space, so as to maintain desirable levels of CO₂ (or other gases). In this embodiment, the control system now controls the rate of exhaust air and outside air to either a preset minimum. If the capacity and efficiency of the scrubber is insufficient to handle the CO₂ load, then the rate of exhaust air and outside air can be set to a higher level. The oxygen flow is separately controlled to maintain the target level of oxygen in the occupied space. Both the control of the exhaust air valves and the oxygen inflow can be subject to a simple feedback loop, with a proportional-integral-differential (PID) algorithm with upper and lower set points. The coupling of the oxygen concentrator to the air flow manifold can be done using any tube of duct fitting, with or without a control valve and/or a flow meter.

The system can be designed in a modular way so that it can be retrofitted on a pre-existing or pre-designed HVAC system. This will enable the benefit of this invention in buildings that already have HVAC systems, with relatively lower costs. The oxygen concentrator and scrubber, with a control system, can be installed and connected to a conventional HVAC system without having to replace the ductwork or the central air handling unit.

Having described an example embodiment, it should be apparent to those skilled in the art that the foregoing is merely illustrative and not limiting, having been presented by way of example only. Numerous modifications and other embodiments are with the scope of ordinary skill in the art and are contemplated as falling with the scope of the invention.

Claims

1. A system for controlling temperature of air in an enclosed environment, comprising:

an inlet configured to receive air from outside the enclosed environment;

an air handling unit connected to receive outside air through the inlet and circulated air from the enclosed environment and configured to affect the temperature of the received air;

an air circulation system configured to circulate air from the air handling unit to designated spaces in the enclosed environment and back to the air handling unit;

a gas scrubbing system configured to reduce the level of unwanted gases in the circulating air.

2. The system of claim 1, further comprising a control system responsive to measurements of composition of the circulated air to control the gas scrubbing system to maintain a desired composition of the circulated air.

3. The system of claim 2, wherein airflow through the inlet is such that the desired air composition is maintained with a lower amount of outside air than would be possible without the gas scrubbing system.

4. The system of claim 1, wherein the unwanted gases includes carbon dioxide.

5. The system of claim 4, wherein the unwanted gases further include at least one of volatile organic compounds, carbon monoxide, nitrous oxides and sulfur oxides.

6. The system of claim 1, wherein the gas scrubbing system is connected to the air circulation system to intercept at least a portion of the circulating air prior to the circulating air reaching the air handling unit.

7. The system of claim 1, wherein the gas scrubbing system is connected to the air the air circulation system to intercept at least a portion of the circulating air after the circulating air is processed by the air handling unit.

8. The system of claim 1, wherein the gas scrubbing system includes a molecular sieve and the unwanted gas is reduced by adsorption of the unwanted gas onto the molecular sieve.

9. The system of claim 8, wherein the molecular sieve is made of a form of zeolite.

10. The system of claim 8, wherein the gas scrubbing system includes at least one additional adsorbent.

11. The system of claim 10, wherein the additional absorbent includes at least one of activated charcoal, silica gel or porous alumina.

12. The system of claim 10, wherein the additional adsorbent is placed in a bed that intercepts flow of circulating air.

13. The system of claim 10, wherein the gas scrubbing system comprises a plurality of beds, wherein each bed intercepts flow of circulating air, and each bed includes an additional adsorbent.

14. The system of claim 1, wherein the gas scrubbing system comprises a system for controlling a reversible chemical reaction.

15. The system of claim 14, wherein the reversible chemical reaction is a sodium carbonate and sodium bicarbonate cycle.

16. The system of claim 14, wherein the reversible chemical reaction is an amine-gas cycle.

17. The system of claim 1, where the gas scrubbing system utilizes one or more bases.

18. The system of claim 17, wherein the base is an alkaline hydroxide.

19. The system of claim 1, wherein the gas scrubbing system is a temperature swing adsorption system.

20. The system of claim 1, wherein the gas scrubbing system includes a purge cycle, wherein a purge gas is applied to the gas scrubbing system to release the unwanted gas from the gas scrubbing system.

21. The system of claim 20, wherein the purge gas is heated by taking heat from a component of a heating, ventilation and air-conditioning system incorporating the air circulation system.

22. The system of claim 20, further comprising a heating system configured to heat the purge gas.

23. The system of claim 22, wherein the heating system uses solar energy.

24. The system of claim 1, wherein the gas scrubbing system comprises an adsorbent, and the system further comprises a cooling system that cools the adsorbent, wherein the cooling system uses a chilled fluid provided by the air handling unit.

25. The system of claim 1, wherein the gas scrubbing system is connected to the air circulation system such that a part of circulating air flows through the gas scrubbing system and another part of the circulating air bypasses the gas scrubbing system.

26. The system of claim 1, further comprising an oxygen injection system that injects oxygen concentrated air into the circulating air.

27. The system of claim 26, further comprising a control system responsive to measurements of oxygen level in the circulating air to control the oxygen injection system so as to maintain a desired level of oxygen in the circulating air.

28. The system in claim 26, wherein the oxygen injection system comprises a pressure swing adsorption or vacuum swing adsorption system.

29. A process for controlling temperature of air in an enclosed environment, comprising:

receiving air from outside the enclosed environment and circulating air from the enclosed environment;

conditioning the received air so as to provide air at a desired temperature;

circulating the conditioned air into and from designated spaces in the enclosed environment;

scrubbing the circulated air from the enclosed environment to reduce unwanted gases in the circulated air;

recirculating the scrubbed air; and

exhausting a portion of the circulated air from the enclosed environment.

30. The process of claim 29, wherein the scrubbed air is conditioned to the desired temperature prior to being recirculated.

31. A control system for use with an HVAC system having a gas scrubbing system for removal of an unwanted gas from circulated air, the control system comprising:

a sensor for determining an amount of the unwanted gas in the circulated air;

a controller that modifies a rate of exhaust of circulating air and intake of outside air so as to adjust overall air replacement according to the measured amount of unwanted gas in the circulated air.