Ammonia-water air-cooled solar chiller

2.1. Thermodynamic design

The proposed prototype is a so-called single effect cycle that uses ammonia as refrigerant and water as absorbent. This working pair also does not bear a crystallization risk as may occur in lithium bromide cycles, and no freezing at sub-zero conditions as occurs to all cycles using water as refrigerant (lithium bromide, silica gel and zeolite chillers). Another advantage is the continuous operation, without charge-discharge cycles (that occur for example in solid sorption or triple phase absorption) with consequent performance fluctuations and cycling losses. The ammonia cycle works at relative high pressure and therefore it does not need purging of non-condensable gases (vacuum maintenance). Assembly is also less critical and costly than for water-based cycles, since it does not need to be vacuum-proof. Ammonia has no significant ozone depletion or global warming potential.

The single effect is the basic working cycle in the absorption technology. It incorporates four main heat exchangers, a solution heat exchanger, a pre-cooler, a rectifier, a solution pump and two throttle valves. The main design task is centred on the four main pieces of equipment within which the entire process takes place, except for the refrigerant and solution transfers between the different pressure levels of the chiller and for the heat exchange in the solution heat exchanger. The basic, single effect cycle is shown in Figure 1. The main components are the evaporator, condenser, absorber, generator, rectifier, pre-cooler, and solution heat exchanger. A recirculation loop connects absorber and generator. In this loop, the working solution is re-circulated. The refrigerant vapour, i. e. ammonia, is boiled off in the generator by heating the working solution (driving heat from the sun). The solution becomes poor in refrigerant content. The refrigerant vapour flows to the condenser where it is condensed releasing waste heat and is then throttled to the evaporator. There, low-temperature heat (from the building) is dissipated to evaporate the condensate refrigerant. Finally, the low-pressure refrigerant vapour enters the absorber and is absorbed into the

poor (diluted) solution throttled from the generator. This process releases additional waste heat. The resulting concentrated solution is pumped back to the generator to close the solution loop. Normally, condenser and absorber produce heat at the same temperature, which is released into the ambient air; in a dual-use heat pump/chiller, the waste heat could be used either for domestic hot water (DHW) production or for space heating in winter. Given that the energy balance over the complete cycle must be nil, the total waste heat released is equal to the total heat required, i. e. the sum of the driving heat (solar) and of the low-temperature heat (cooling). The ratio between cooling and driving heat, i. e. the benefit vs. the effort, is the coefficient of performance (COP) of the chiller. Furthermore, precooler and solution heat exchanger help reducing thermodynamic losses in the cycle, thus enhancing the COP.

Подпись: Fig. t.Basic flow scheme of the single effect ammonia-water absorption chiller with indication of heat streams.

Ammonia as refrigerant reaches at the required operational temperatures a quite high vapour pressure (between 400 and 2000 kPa), which enables for a very compact construction of the heat exchangers. Historically, shell and tubes or tube-in-tube heat exchangers are utilized in the small capacity range. Recently, also plate heat exchangers have been tested in laboratory prototypes. On the refrigerant side, a pipe with throttle valve connects the condenser with the evaporator, like in the well-known recompression chillers. A throttle valve is also needed in the solution circuit.

Подпись: Fig. 2. Ammonia water chiller under development

Finally, an electrically driven solution pump is used to circulate the working solution. Its power consumption can reach values from 3 % up to 10 % of the cooling capacity. It is therefore mandatory for a solar-based chiller to find a very efficient and reliable pump for a satisfactory and energy-saving operation. An image of the first prototype built at Ao Sol is depicted in Figure 2. Abundant empty space was foreseen in the inner volume in order to carry out typical laboratory work such as changes and repairs.