Summer is hot . That is probably an understatement considering that most years we get temperatures in excess of 100 degrees for days on end and long droughts. Survival for most people includes hiding out in an air conditioned house watching t.v. and sipping sweetened iced tea. For us, however, it means eating ice, getting chores done in the less hot morning, and accepting that our lives change with the seasons. Our house is just too old and big to be able to afford conventional air conditioning.
Of course we run fans, ceiling fans and such. The benefit of an older home is that they were designed to stay cool without the air conditioner using good cross ventilation and tall, narrow windows. The heat rises to the tops of the high ceilings and, for the most part it is livable for us. However, if there were an alternative I would not be adverse to it!
Adsorption chillers use solid sorption materials instead of liquid solutions. Market available systems use water as refrigerant and silica gel as sorbent; but recently, an alternative to silicagel is zeolith for some manufacturers. So the two technologies now available are: Silicagel/H2O and Zeolith/H2O.
The machines consist of two sorbent compartments (see figure) – one evaporator and one condenser. While the sorbent in the first compartment is regenerated using hot water from the external heat source, e.g. the solar collector, the sorbent in the second compartment adsorbs the water vapour entering from the evaporator. Compartment 2 has to be cooled in order to enable a continuous adsorption. Due to the low pressure conditions in the evaporator, the refrigerant in the evaporator is transferred into the gas phase by taking up the evaporation heat from the chilled water loop and thereby producing the useful "cold". If the sorption material in the adsorption compartment is saturated with water vapour to a certain degree, the chambers are switched over in their function.
To date, only a few Asian and European manufacturers produce adsorption chillers. The two historical actors are Japanese, but now a German manufacturer has entered the market.
Under typical operation conditions with a driving temperature of 80 °C, the systems achieve a coefficient of performance (COP) of about 0.6, but operation is possible even with temperatures of approx. 60 °C. The capacity of the chillers ranges from 5.5 kW to 500 kW chilling power.
The simple mechanical construction of adsorption chillers and their expected robustness is an advantage. There is no danger of crystallisation and thus no limitation in temperatures. There is no internal solution pump and electricity consumtion is reduced to a minimum. A disadvantage is the comparatively large volume and weight. Furthermore, due to the small number of produced items, the price of adsorption chillers is currently still high. A large potential for improvements is expected in the construction of the heat exchangers in the adsorber compartments, which would reduce volume and weight considerably in future generations of adsorption chillers.
Desiccant cooling systems are basically open cycle systems, using water as refrigerant in direct contact with air. The thermally driven cooling cycle is a combination of evaporative cooling with air dehumidification by a desiccant, i.e. a hygroscopic material. For this purpose, liquid or solid materials can be employed. The term ‘open’ is used to indicate that the refrigerant is discarded from the system after providing the cooling effect and new refrigerant is supplied in its place in an open-ended loop. Therefore only water is possible as refrigerant with direct contact to the surrounding air. The common technology applied today uses rotating desiccant wheels, equipped either with silica gel or lithium-chloride as sorption material.
Solid Desiccant Cooling
The main components of a solar assisted desiccant cooling system are shown in the figure on the right. The basic process in providing conditioned air may be described as follows.
Warm and humid air enters the slowly rotating desiccant wheel and is dehumidified by adsorption of water (1-2). Since the air is heated up by the adsorption heat, a heat recovery wheel is passed (2-3), resulting in a significant pre-cooling of the supply air stream. Subsequently, the air is humidified and thus further cooled by a controlled humidifier (3-4) according to the set-values of supply air temperature and humidity. The exhaust air stream of the rooms is humidified (6-7) close to the saturation point to exploit the full cooling potential in order to allow an effective heat recovery (7-8). Finally, the sorption wheel has to be regenerated (9-10) by applying heat in a comparatively low temperature range from 50 °C-75 °C and to allow a continuous operation of the dehumidification process.
Solid desiccant systems can also be used to provide heating for periods with low heating demand.
Flat-plate solar thermal collectors are normally applied as heating system in solar assisted desiccant cooling systems. The solar system may consist of collectors using water as fluid and a water storage, which will increase the utilisation of the solar system. This configuration however requires an additional water/air heat exchanger, to connect the solar system to the air system.
Special design of the desiccant cycle is needed in case of extreme outdoor conditions such as e.g. coastal areas of the Mediterranean region. Due to the high humidity of ambient air, a standard configuration of the desiccant cooling cycle is not able to reduce the humidity down to a level that is low enough to employ direct evaporative cooling. More complex designs of the desiccant air handling unit employing for instance another enthalpy wheel or additional air coolers supplied by chilled water can overcome this problem. A novel approach is the dehumidification and simultaneously cooling of the supply air in an air-to-air heat exchanger, in which the supply air is dehumidified through sorptive coatings at the heat exchanger wall, and cooled by the returned air, which was humidified close to saturation in order to lower the return air temperature before entering the heat exchanger. The simultaneously dehumidification and cooling improves the efficiency of the system. As a consequence, the supply air humidification may be avoided in moderate climates. Since the sorption material in the supply side of the heat exchanger will be saturated after some time, a periodic operation with two heat exchangers of which one is regenerated, is required. A pilot project in Germany for testing this new concept is currently in the design phase.
Liquid Desiccant Cooling
A new technology, close to market introduction, are desiccant cooling systems using a liquid water-lithium chloride solution as sorption material. This type of systems shows several advantages like higher air dehumidification at the same driving temperature range of solid desiccant cooling systems, and the possibility of high energy storage by storing the concentrated solution. This technology is a promising option for a further increase in exploitation of solar thermal systems for air conditioning. Currently, a few systems of this type are installed in Germany in pilot and demonstration applications, driven either with solar thermal heat or from other heat sources.
Absorption chillers are the most distributed thermally driven chillers worldwide. A thermal compression of the refrigerant is achieved by using a liquid refrigerant/sorbent solution and a heat source, which is replacing the electric power consumption of a mechanical compressor. For chilled water above 0 °C, as it is used in air conditioning, typically a liquid H2O/LiBr solution is applied with water as refrigerant. Nevertheless, other liquid solutions can be used like H2O/LiCl or NH3/H2O which permits to produce chiled water at temperatures below 0 °C.
The main components of an absorption chiller are the generator, the condenser, the evaporator and the absorber (see figure on the right).
The cooling effect is based on the evaporation of the refrigerant (water) in the evaporator at very low pressure. The vaporised refrigerant is absorbed in the absorber, thereby diluting the H2O/LiBr solution. To make the absorption process efficient, the process has to be cooled. The solution is continuously pumped into the generator, where the regeneration of the solution is achieved by applying the driving heat such as from hot water supplied by a solar collector. The refrigerant leaving the generator by this process condenses through the application of cooling water in the condenser and circulates by means of an expansion valve again into the evaporator.
Many of these products are available in the market, however typical chilling capacities of absorption chillers are several hundred kW. For several years, the smallest machine available was a Japanese product with 35 kW capacity. Mainly, they are supplied with district heat, waste heat or heat from co-generation. The required heat source temperature is usually above 80 °C for single-effect machines and the coefficient of performance (COP) is in the range from 0.6 to 0.8. Double-effect machines with two generator stages require driving temperature of above 140 °C, but the COP’s may achieve values up to 1.2.
Since a few years, several single-effect absorption chillers with capacities below 50 kW are available. In SAC systems with absorption chillers, these small units are now often implemented. A chiller model, newly developed for small capacities, enables part-load operation with reduced chilling power at a heat source temperature of 65 °C and a COP of still approximately 0.7, which is very promising in combination with a solar heat source. This shows that there is a high potential for performance improvements of absorption chillers.The new medium-size and small-size developments have been designed recently by European and Asian manufacturers, convenient to cover the cooling loads for small areas such as from 200 m² to 500 m². The European manuacturers are located in Germany, Austria, Spain, Sweden, Italy and Portugal. Some of the developments are still being tested in pilot installations.
