Chris Bales, Hogskolan Dalarna, Solar Energy Research Center (SERC) 78188 Borlange, Sweden. e-mail:cba@du. se

Fredrik Setterwall, Fredrik Setterwall Konsult AB, Backvagen 7c, 192 54 Sollentuna, Sweden, e-mail: fredrik. setterwall@comhem. se

Goran Bolin, ClimateWell AB, Instrumentvagen 20, 12653 Stockholm, Sweden. email: goran. bolin@climatewell. com

The Thermo Chemical Accumulator (TCA) is a chemical heat pump driven by low temperature heat that has integral heat storage with high energy density. This makes the device very suitable for solar cooling. The working pair consists of Lithium Chloride and water, and energy is stored and released by desorption and absorption of water under near vacuum conditions. In contrast to most absorption processes and chemical heat pumps, the TCA works with three phases: solid, solution and vapour. This results in near constant operating conditions during charge and discharge, independent of state of charge. This paper describes the fundamental working principles of the TCA as well as a simple steady state model for the TCA. A temperature difference between theoretical and effective temperature in the reactor during absorption and desorption was required in order to get reasonable agreement with measurement data of a prototype TCA machine. For absorption, this value for subcooling was 15°C, which is significantly higher than has been found for low-temperature absorption chillers, indicating potential for improvement. For desorption the value was 7.5°C. The TCA has desorption temperatures of below 100°C for ambient temperatures below 40°C, which is relatively low. The temperature lift depends on the cooling rate supplied and varies from 15°C for the design cooling rate of 5 kW per TCA unit and 30°C inlet temperature to the reactor, to 20°C for a cooling rate of 2.5 kW. The energy density for storage was 180 kWh/m3 for the tested prototype.

Background

In many countries there is a continual increase in the demand for comfort cooling of buildings. This is reflected in the increase in the number of room air conditioners sold in southern Europe, as exemplified in Greece where the number of units sold more than doubled to 200000 units between 1990 and 2000 (Papadopoulos et al., 2003). Most of these were small units, with over 70% having a cooling capacity of less than 3.5 kW. These are predominantly electrically driven compressor heat pumps, resulting in peak electricity demand during the hottest periods and amplifying the heat island effect in large towns where the local ambient temperature is much higher than in surrounding areas. These units also use refrigerants that are either damaging to the ozone layer or are powerful greenhouse gases or both. Due to recent EU regulations regarding the phasing out of ozone depleting substances (2000), the cost of owning units with such substances will increase.

There are thus several reasons to develop alternatives to the vapour compression heat pump for providing comfort cooling: reduction of the peak electricity load during hot periods; replacement of refrigerants that are ozone depleting and/or strong greenhouse gases; and reduction of the heat island effects in large towns. In order to reduce the electricity load, thermally driven cooling processes have been developed and some have
been applied commercially. The commercial units use either absorption or adsorption cycles.