T.X. Li

High temperature hot water heat pump with non-azeotropic refrigerant mixture HCFC-22/HCFC-141b

A water-to-water high temperature heat pump was studied experimentally. The performance of the system was characterized by refrigerant compositions, compressor RPM and water temperature change. For the experimental conditions of the inlet water temperature of evaporator of 40 °C and the inlet and outlet water temperatures of the condenser of 70 and 80 °C, respectively, the experiment shows that the coefficient of performance is maximum when the molar component of R22 is about 75%. It is shown that the maximum pressure of the system is under 2.5 MPa after taking R22/R141b as working fluids, even though the highest cooling water temperatures is about 80 °C.

Enhancement of Heat and Mass Transfer in Solid Gas Sorption Systems

Solid gas sorption systems driven by heat have gained much attention due to their energy conservation and environmental benefits. These sorption machines can be driven by waste heat or renewable energy source such as solar energy, and can utilize natural working fluids with no GWP and ODP, such as water, methanol, and ammonia. However, poor heat transfer process and slow diffusion rate of the refrigeration gas in the adsorber have been identified as the main drawbacks limiting the cooling density performance, and consequently, commercialization of sorption machines. This paper provides a review of techniques that have been applied to enhance heat and mass transfer in solid gas sorption systems. These techniques mainly include the use of materials with high thermal conductivity, consolidation of adsorbents, and the use of specially designed heat exchangers in the adsorbers. The effect of these methods on the coefficient of performance and the specific cooling power is also discussed.

Heat Transfer Design in Adsorption Refrigeration Systems for Efficient Use of Low Grade Thermal Energy

Adsorption refrigeration and heat pump systems have been considered as very important means for the efficient use of low grade thermal energy in the temperature range of 60–150°C. Sorption systems are merely heat exchanger based thermodynamic systems, and therefore a good design to optimize heat and mass transfer with reaction or sorption processes is very important for high performance of the systems. Studies on heat and mass transfer enhancement in adsorption beds have been done extensively. Notable techniques is whereby the adsorbent bed is fitted with finned heat exchanger embedded with adsorbent particles, or the adsorbent particles may be compressed and solidified and then coupled with finned tube or plate heat exchangers. The use of expanded graphite seems to be an effective method to improve both heat and mass transfer in the reaction bed. Studies have also shows the need to enhance the heat transfer in adsorption bed to match with the heat transfer of thermal fluids. Use of heat pipes and good thermal loop design could yield higher thermal performances of a sorption system, when coupled with adsorption beds to provide heating and cooling to the beds. A novel design with passive evaporation, known as rising film evaporation coupled with a gravity heat pipe was introduced for high cooling output. It has also been shown that heat and mass recovery in the internal sorption systems is critical, and novel arrangement of thermal fluid and refrigerant may result in high performance sorption systems. Based upon the above researches, various sorption systems have been developed, and high efficient performances have been reached. Typical sorption systems include (1) A silica gel-water adsorption water chillier with a COP about 0.55 when powered with 80°C hot water, (2) A CaCl2 -ammonia adsorption refrigerator with a COP over 0.3 at −20 °C when powered with 120 °C water vapor, which has a specific cooling power about 600 W/kg-adsorbent. The above mentioned systems have shown that solid sorption systems have become market potential products, and low grade thermal energy, which is usually considered as waste heat, could be utilized to provide high grade cooling. This paper gives details of high efficient solid sorption systems recently developed, their heat transfer design, thermodynamic system coupling, and performance test results. Some examples of low grade thermal powered cooling systems are also presented.Copyright

Thermochemical heat storage for solar heating and cooling systems

Advanced energy storage is an essential key technology for adjusting the time discrepancy and instability between solar energy supply and energy demand in solar heating and cooling systems. Among the various energy storage technologies, the thermochemical energy storage method has received more attention because of its distinct advantages of higher energy storage density and low heat loss compared with the conventional sensible and latent energy storage methods. The development of thermochemical energy storage technology is reviewed in this chapter. A lot of theoretical and experimental studies on reactants, chemical reaction kinetics, and system designs are introduced and analyzed, and the future development of thermochemical heat storage is discussed.

Progress in Sorption Thermal Energy Storage

There are various ways for thermal energy storage, such as sensible, latent, sorption, and chemical reaction. Sensible thermal energy storage and latent thermal energy storage are already in use. However, the drawbacks of bulk size (small energy storage density) and the strict requirement for thermal insulation have hindered their wide applications. Sorption and thermochemical reactions used for thermal energy storage have been considered as a future great potential product for thermal energy storage of solar energy, waste heat. or even electric heating, etc. The market thus needs such a “thermal battery,” which should be with a variety of kWhs capacities. Several key challenges remain in the way of the development of an efficient sorption thermal battery: sorption materials with high storage density and low cost, sorption bed with good heat and mass transfer to ensure charging power and discharging power, being stable after repeated cycles, minimum heat capacity ratio between the inert materials to the sorption thermal energy; control of the output temperature and power to meet the use demand. In this chapter, recent progress in sorption thermal energy storage, including materials, systems, and demonstrations, were described. The detailed future researches and developing maps were also discussed.