Space Cooling  »  Thermally driven and solar air conditioning

In closed thermodynamic cycle cooling is done through evaporation of refrigerant in near vacuum conditions created by the process of ad/absorption. Systems using closed thermodynamic cycles are of two categories. They are absorption chillers and adsorption chillers. Both form of chillers have similar sub systems with the only difference being the process (absorption or adsorption) and materials used for sorption. A solar cooling sorption system requires three basic sub-systems (Becker, Helm and Schweigler, 2009):

• A solar thermal system to produce the require heat. Typical in the form of hot water and steam
• A thermal chiller and chilled water distribution system
• A heat rejection system

There are five main components to a solar cooling system as show in the figure above:

• Solar thermal collectors: These are orientated towards the sun and are usually placed on the roofs of buildings, but can also be ground-mounted if the space is available. The heat energy from the Sun is used to heat up the heat transfer fluid.
• Heat storage/cold storage: systems typically have some heat storage or cold storage. A heat store can act as a buffer for the system and in some cases to ensure cold production when conditions mean that the solar field won’t provide enough energy to meet cooling loads. It also can allow smaller chillers by allowing peak cooling loads to be met from storage, rather than having expensive chiller capacity idle in most operating conditions.
• Pumps, controls and other balance of plant: are required to circulate the heat transfer fluid, optimise operation and ensure the proper functioning of the system. An optimised control system is critical to minimising the cost of cold production.
• Thermally driven chiller or desiccant system: Are a proven technology, but are less efficient than electrically driven vapour-compression cycles, with COPs rarely exceeding one. There are three main types: adsorption chillers, absorption chillers and desiccant systems.
• Heat rejection unit: Is required to reject the heat or the cold of the thermally driven chiller and the cold storage system respectively. These can be air coolers, cooling towers, geothermal loops, swimming pools, etc.

The solar thermal system provides the heat to the desorber (G) of the chiller, while heat is rejected from the condenser (C) and absorber or adsorber (A) via the cooling water loop and cooling tower. The cooling is provided by a chilled water loop from the evaporator (E) (Becker, Helm and Schweigler, 2009). The system can include hot and or cold storage in the form of hot water, cold water or ice storage. The specific ambient conditions, demand characteristics and goals for a system mean that a variety of technical options need to be selected to meet all operating and cost goals. Detailed description, advantages, applicability and other information can be found in the following sections, absorption chiller and adsorption chiller. 

Open thermodynamic cycles in simple terms can be equated to evaporative cooling. The refrigerant in this process is lost to atmosphere and hence it is called open cycle. One of the popular forms of open cycle solar air conditioning is desiccant evaporative cooling (DEC). In desiccant evaporative cooling process air is dehumidified by a passing it over a solid or liquid desiccant solution. The desiccant absorbs/adsorbs the humidity in the air making it dry and allows direct evaporative cooling through which it is cooled (also humidified). Solar thermal energy provides the heat that allows the desiccant to be regenerated (dried out). Besides, solar heat, biomass gasifier or waste heat can also be used for this purpose. The advantage of these systems is that they require low-grade heat and avoid the need for expensive chillers.

The challenge however, is that, although desiccant systems are quite efficient in dealing with the latent load (put simply, moisture removal), they are much less efficient in meeting the sensible load (the cooling of the air). Solar driven desiccant evaporative systems, depending on circumstances and expected ambient conditions, may therefore be best paired with electric powered chillers to deal with the remaining sensible heat load that isn’t met by desiccant evaporative system. In this case, the electric heat pump can operate at significantly higher evaporator temperatures, significantly improving efficiency (by reducing the temperature differential to be bridged) and making the overall system much more efficient and less energy intensive.

Absorption chillers work on the principle of sorption/desorption and require heat inputs in the range of 70 °C to 180 °C to drive the chillers. The most common combinations of sorption material/refrigerant are water/ammonia and lithium-bromide/water. First the refrigerant (typically ammonia or water) is absorbed in the absorbent (typically water or LiBr solution), which decreases it’s vapour pressure and the vapour pressure is increased by desorption. The absorption cycle was invented in 1846 by Ferdinand Carré to produce ice using a heat input. They were used extensively in early part of the 20th century to run gas fired refrigerators and ice machines. Typical efficiencies are in the range of 0.5 to 0.8 for single effect machines and up to 1.4 for double effect machines that require higher driving temperatures. Primary energy savings of 30% to 50% (compared to an electric chiller) should be possible when space cooling is provided through waste heat. Absorption chillers consist of four major compartments. They are Generator, Condenser, Evaporator and Absorber.  In order to drive the systems it also requires some basic components namely: heat source (e.g., solar thermal collectors, natural gas, waste heat etc.), heat storage/cold storage, pumps, controls and other equipment.

Generator (desorption happens here)
A solution of refrigerant and absorber is heated in vacuum present in the generator chamber. This boils the solution and increases the temperature and vapour pressure of the refrigerant, which then separates from the absorber liquid leaving behind a concentrate solution, which is pumped, into the absorption chamber. (In loose terms this can be related to the work done by the electrical compressor in the vapour compression cycle to increase the temperature and pressure of the vapour).

Condenser (Condensation)
Once separation occurs, the high temperature, high-pressure refrigerant vapour flows up to the condenser. In the condenser, the refrigerant vapour is condensed on the surface of the cooling coil and latent heat due to condensation is removed. The latent heat is rejected to a cooling tower through cooling coils. This low temperature, high-pressure refrigerant liquid accumulates in the condenser and then passes through an orifice into the evaporator (Similar to the conden-ser in vapour compression cycle). 

Evaporator (Evaporation occurs here)
In the evaporator, the refrigerant liquid passes from high pressure to low pressure because of the expansion valve, increases in volume and is exposed to deeper vacuum than in the con-denser. In such low-pressure situations the boiling point of refrigerant decreases tremendously and it evaporates at around 4 C. This evaporation causes cooling, which is transmitted through a heat exchange between the evaporator and the surrounding air or chilled water coils present in the evaporator. The resulting low temperature and low pressure refrigerant vapour is then at-tracted to the absorber. 

Absorber (Absorption happens here)
A deep vacuum in the absorption chamber is maintained by the affinity of the concentrated so-lution pumped from the generation chamber (left behind from the first step – generator) with the refrigerant vapour formed in the evaporator. The refrigerant vapour is then absorbed by the concentrated absorber solution flowing across the surface of the absorber coil. Heat from the condensation and dilution are removed by the cooling coils and rejected to a cooling tower. The resulting dilute solution is pre-heated in a heat exchanger before returning.

Types of absorption chillers

Adsorption closed cycle chillers require a driving heat source as low as 55°C and up to 90°C, but typically around 75°C for small-scale chillers. In adsorption chillers a refrigerant (typically water) is adsorbed into a solid sorbent (typically a zeolite, silica gel or activated alumina) and can generate cold water as low as 5°C. Primary energy savings of 30% to 50% are possible when the system is used for air conditioning from renewable or waste heat sources. Adsorption chillers consist of four major compartments, they are, Generator, Condenser, Evaporator and Adsorber. In order to drive the systems it also requires some basic components namely: heat source (e.g., solar thermal collectors, natural gas etc.), heat storage/cold storage, pumps, controls and other equipment. Working of an adsorption chiller is described below.

Adsorption closed cycle chillers require a driving heat source as low as 55°C and up to 90°C, but typically around 75°C for small-scale chillers. In adsorption chillers a refrigerant is adsorbed into a solid sorbent (typically a silica gel) and can generate cold water as low as 5°C. Primary energy savings of 30% to 50% are possible when the system is used for air conditioning from renewable or waste heat sources. Adsorption chillers consist of four major compartments, they are, Generator, Condenser, Evaporator and Adsorber. In order to drive the systems it also requires some basic components namely: heat source (e.g., solar thermal collectors, natural gas etc.), heat storage/cold storage, pumps, controls and other equipment. Working of an adsorption chiller:

Generator (desorption happens here)
The saturated adsorbent is dried by heat input. This increases the vapour pressure of the trapped water in the adsorbent. The water vapour is set free and it flows into the condenser as a high temperature high-pressure vapour.

Condenser (Condensation)
The high temperature high-pressure water vapour condensates into high-pressure liquid (water) here releasing the latent heat of condensation. The latent heat is rejected to a cooling tower through cooling coils. This low temperature, high-pressure water accumulates in the condenser and then passes through an orifice into the evaporator (Similar to the condenser in vapour compression cycle). 

Evaporator (Evaporation occurs here)
The low temperature, high-pressure water from the condenser is then passed through orifice into the evaporator chamber with near vacuum conditions. In such low-pressure situations the boiling point of water decreases tremendously and it evaporates at around 4 C. This evapora-tion causes cooling, which is transmitted through a heat exchange between the evaporator and the surrounding air or chilled water coils present in the evaporator.

Absorber (Absorption happens here)
The vapour leaves the evaporator at a low-pressure low temperature and the dry adsorbent aspirates this water vapour. During the adsorption process heat is rejected which has to be dis-sipated through the use of cooling tower or by other means similar as it is rejected during con-densation. In a final step, the saturated desiccant is set back for desorption through heating and the cycle continues.

Desiccant cooling and desiccant evaporative cooling (DEC) open cycles are thermally driven air conditioning processes that are based on the combination of sorptive dehumidification and evaporative cooling. They are “open” cycles in that the refrigerant (water) is discarded from the system after use and replaced by new water. Heat is required to regenerate (dry out) the sorptive material which can be a liquid or a solid. The required driving heat is low, between 50°C and up to 100°C depending on the system design and climate. DEC systems, sometimes used in conjunction with thermally driven chillers or standard air conditioners, improve efficiency by dehumidifying and cooling the air using less energy than a standalone air conditioner. There are two types of DEC systems, those using a solid desiccant and those using a liquid desiccant.

Solid desiccants
DEC systems using solid desiccants use either a wheel or fixed bed systems. With the wheel system, a desiccant is housed in a wheel that slowly rotates, the humid process air is passed through the bottom of the wheel, and the humidity is adsorbed by the desiccant and the air passes through drier and warmer (A-B). A second wheel acts as a heat exchanger (B-C) and lowers the process air temperature before the air is passed through an evaporative cooler (C-D) and then into the space (E). On the top side of the system the return air is heated prior to the desiccant wheel and the desiccant is “regenerated” as the water in the desiccant wheel evaporates and is vented to the atmosphere in the exhaust air thereby removing excess humidity from the building. Conventional air conditioning may still be required to achieve the desired air temperature, but the removal of the latent heat load greatly reduces the energy needs to achieve this.

Solid desiccants are chemical compounds and include synthetic polymers, natural or synthetic zeolites, silica gels, titanium silicates, and activated aluminas. They act as very efficient sponges, because their internal surface area per unit of volume is huge.The standard DEC cycle in hot humid climates won’t be able to meet all cooling requirements, because of the high latent loads. This can be addressed by using auxiliary cooling and de-humidification within the DEC system or by using conventional air condi-tioning in series with the DEC.

Liquid desiccants
With liquid desiccants, a loop where the absorber and regenerator devices are linked results in the dehumidification of the air and then the regeneration of the desiccant in the regenerator.  In the dehumidification process, the liquid desiccant absorbs the water vapour and releases heat, which is then carried away to a heat sink (typically a cooling tower). As the water vapour is absorbed from the ambient air it dilutes the liquid desiccant, reducing the effectiveness of the solution. The desiccant is then passed to a regenerator where thermal energy heats the liquid desiccant to a temperature where the water vapour desorbs, “regenerating” the liquid desiccant, and is then carried away by the exhaust air stream.

Dehumidification can be controlled by varying the concentration of the liquid desiccant that is supplied to the device or by decreasing the flow of highly concentrated desiccant. The latter approach has a similar effect, because the highly concentrated desiccant is quickly diluted and thus performs as a weaker desiccant solution.

Low-flow liquid desiccant systems, where the desiccant flow is varied to ensure only the minimum necessary to remove the current moisture content of the air is removed, reduce the flow rate and allow low pressure drops, avoiding some of the problems with high flow systems. Various options are possible for the regenerator heat requirement. Solar panels can achieve solar conversion efficiencies of 60% to 120% with the above system, while if using natural gas or high-grade heat, latent COPs of 0.7 to 0.8 are possible, and between 1.1 and 1.4 if a two-stage regeneration system is used (Kozubal et al., 2011). Using solar panels makes the system dependent on available solar radiation, to a greater or lesser extent depending on the level of storage incorporated, with the key consideration being the relative economics of the heat source.

The heat required to complete the first step in the generator could be obtained from gas fired, electrical, waste heat available or solar powered sources. Most thermal chillers use renewable heat energy from solar powered sources (including biomass gasifier) to the extent possible and thus depending less on conventional fossil fuels or electricity. Therefore, these systems are generally referred to as solar cooling systems. In countries with lower COPs for new air conditioners and less efficient electricity production than the global average, the case for China and India for instance, thermal chillers can save significant amounts of primary energy. The real benefits of using thermally driven chillers comes from the very large CO₂ emissions reductions that are possible in countries with CO₂ intensive electricity generation systems, particularly, where renewable heat sources or waste heat is available to drive the chiller.

The efficiency (COP) of single-effect absorption chillers is typically between 0.5 and 0.75, significantly lower than that of vapour-compression systems which are typically in the range 3 to 3.8 for new units. Adsorption chillers don’t have the opportunity to use double-effect configurations, so COPs of one or greater will not be achieved in the short to medium-term. It should be noted that comparing the COPs of thermally driven chillers and vapour-compression using electricity, as the driving energy isn’t comparing like-for-like, as the efficiency of electricity production globally is just 40%. Thermally driven chillers have lower COPS, but use heat (solar, gas, waste heat, biomass, etc.) directly. A comparison of the primary energy use for a unit of cold production therefore gives a more valid indication of the efficiency of the two options at an overall system level. Assuming a primary energy factor of 2.8 for delivered electricity means that vapour-compression air conditioners with a COP of 3 consume around one unit of primary energy per unit of cold produced. Assuming that a thermally driven chiller is used for only cooling and it has a COP in the range 0.5 to 0.75 would mean that 1.3 to 2 units of primary energy would be required to produce one unit of cold. Thus a thermally driven chiller would require more final and primary energy to produce a unit of cold than a standard vapour-compression air conditioner. However, when using waste heat or heat from renewable energies, it will save fossil primary fuels such as oil or coal.

Absorption chillers
The efficiency (COP) of absorption chillers ranges from 0.5 to 0.8 for single-effect machines and require heat source temperatures of around 80 °C on average. Double-effect machines have COPs of between 1 and 1.4 with average heat source requirements of 130 °C. This heat required to complete the first step in the generator could be obtained from gas fired, waste heat available or solar powered sources. Most absorption chillers use renewable heat energy from solar powered sources (including biomass gasifier) to the extent possible and thus depending less on conventional fossil fuels or electricity. When used in conjunction with solar water heaters these systems are generally referred to as solar cooling systems. The temperature increase in the heat source is significant, as not all solar types of collectors can provide useful temperature of 130 °C and this can increase capital costs.

Compared to this, COP of an electric chiller is in the range of 3 to 6. However, this isn’t a valid comparison, because vapour-compression air conditioners use electricity, as the driving energy and the efficiency of electricity production globally is low. A better metric would be Resource COP which takes into account source to site efficiency of the fuel and transmission losses.

Comparison of resource COP between an electric chiller and an absorption chiller

The principal opportunities for improving the efficiency of absorption chillers comes from using double-effect machines to achieve COPs of up to 1.4, but these require higher driving temperatures in the range 130 °C to 160 °C and require highly efficient evacuated tubes or concentrating parabolic collectors to generate hot water at those temperatures. Multi-stage double or even triple effect combinations could raise the COP to 2 or higher, but would require a heat source of 220 °C for a COP of 2 and even higher for higher COP (Hwang, 2010). However, perceived efficiency of absorption chillers over electrical chillers depends both on the resource COP as well as the supply of renewable or waste source of heat energy.

Adsorption chillers
The efficiency (COP) of adsorption chillers ranges from 0.5 to 0.75 for single-effect machines. They have the advantages of requiring heat source temperatures (for desorption) of as little as 55°C and up to 90°C. They also have no moving parts and have a nearly silent operation apart from a refrigerant pump. In addition, they are simple in design and as a result have significant cost reduction potentials (Henning, 2010). The main drawbacks are their low COPs and higher costs than vapour-compression chillers, particularly for the small size ranges used in residential applications.

Adsorption systems don’t save final energy, as their COPs are significantly lower than standard vapour-compression air conditioning units which typically have COPs in the range of 3 to 6. However, the electricity needed by the latter is often generated and supplied with an amount of primary energy higher by a factor of 2.5 or more, reducing primary energy-related COP equivalents to 1.0 to 2.5. Adsorption chillers produce cold water in closed loop systems than can then be used for space cooling. Efficiencies of the adsorption chillers are lower than vapour-compression systems and range from COP of 0.5 to 0.75. However, in terms of primary energy, adsorption systems may save large amounts of fossil fuels when using waste heat or heat from renewable energy sources.

Desiccant evaporative cooling (DEC)
Thermal COPs (cooling of the DEC, less any auxiliary cooling divided by thermal input) can range between 0.3 and 0.8 for desiccant wheel systems depending on the efficiency of the heat exchanger and the electrical COP (cooling provided divided by the electricity used by the unit) between 2.5 and 3.5. Primary energy savings can range from 30% to 40% if fossil fuels are used as the heat source, and up to as much as 85% in certain climates if solar thermal heat is used compared to conventional air conditioning systems (Beccali, 2010). For liquid desiccant systems analysis for the residential sector in United States suggests that the overall source COP could be as high as 1.4 (Kozubal et al., 2011). Higher efficiencies are achieved with lower inlet temperatures and humidity, as well as cold desiccants. In some cases (high temperature and humidity regions) it maybe necessary to pre-cool the air and pre-dehumidify the air in order to meet peak cooling loads. This will reduce primary energy savings. The efficiency of a DEC system depends on a number of factors including, but not limited to:

• Materials used for the dessicant and their properties
• Temperature and humidity of the outside air
• Desired temperature and humidity of the processed air
• The velocity of the air
• The mass of the desiccant compared to the airflow

Residential sector
To date, no large-scale (> 50 kW) solar cooling systems have been installed in multi-family residential dwellings. There has been growing interest in developing small-scale (up to 30 kW of cooling capacity) thermally driven chillers for the residential market that can be used in single-family or small multi-family dwellings. There are a small but growing number of installations in the residential market, with a number of chiller kits specifically designed for residential applications now on the market from SolarNext, Yazaki and EAW for instance (see table below) (U & S, 2009) (IEA SHC, 2009).

Overview of commercially available absorption chillers with a nominal chilling capacity of < 20kW

Overview of commercially available adsorption chillers with a nominal chilling capacity of < 20kW

Commercial
Absorption chillers for larger applications are available in higher capacities in the range of 50 kW to 5,500 kW. The heat input is in the form of hot water, low pressure steam and high pressure steam. The heat source could be integrated boilers, waste heat or solar energy.

Overview of commercially available large scale absorption chillers with a nominal chilling capacity of upto 5,275 kW