Investigations into solar powered diffusion-absorption cooling machines
Solar thermal driven or assisted absorption cooling machines are gaining increasing importance due to the continually growing demand for airconditioning in domestic housing as well as office and hotel buildings. Presently there are no thermally driven absorption cooling machines available on the market that can provide a smaller cooling capacity than 10kW for cooling buildings. For this reason, two single-effect solar heated ammonia/water (NHJlH20) Diffusion-Absorption Cooling Machines (DACM) with a design cooling capacity of 2.SkW at evaporator temperatures between -10°C and +soC were designed, built and operated. The indirectly heated, solar powered generator (bubble pump) is the main new feature of this cooling machine. Data acquisition was conducted under laboratory conditions as well as under simulated field conditions for vacuum-tube collectors. The first pilot plant showed that coefficient of performance values (COP) range from 0.1 to 0.2 and the evaporator cooling capacity of the pilot plant could reach 1.SkW, but that the operation stability was insufficient. The second optimised and compacted pilot plant showed stable and continuous temperature and pressure levels. The reached COPs were between 0.2 and O.S and the continuous cooling performance between 1.0kW and 1.6kW. A maximum cooling performance of 2.0kW could be reached if the evaporator temperature was set to the relatively high value of 2SoC. The Diffusion-Absorption Cycle has been modelled using the constant characteristic equation of sorption chillers. An expanded, steady-state model which includes additional specific components of the DACM was developed based on the exact solution of the internal mass and energy balances of each component as well as the heat transfer between external and internal temperature levels. The internal enthalpies are calculated at each time interval using the simulation environment INSEL. The compared experimental and simulated data showed a good accordance.