Adsorption Refrigerators and Heat Pumps

Active carbons can be used in both refrigeration and heat pumping cycles, but their potential for use in these applications does not necessarily merit the development of such systems. Before devoting research and development effort into active carbon-based thermodynamic cycles, the interest in both heat-driven cycles in general, and adsorption cycles in particular, must be justified. 

The COP s of specific air conditioners will vary widely with both manufacturer and application. Electricity utility efficiencies will also differ between countries. However, the reason for the economic interest in heat-driven cycles remains clear. Given that primary fuels can cost the consumer approximately 25% of the cost of electricity and that electricity frequently costs more at times of peak demand, there is justification for considering alternative systems. The use of a primary fuel at the point of use can also reduce C02 and other emissions. 

Another reason for the interest in heat-driven cycles is their ability to produce higher temperature outputs than vapour compression cycles. There are industrial 

Bromide as the absorbent in air conditioning applications. It does not suffer from the same problem, since LiBr is effectively non-volatile. However, the pair does have limitations due to the crystallisation limits of LiBr in water. In very hot climates where heat rejection temperatures are higher than about 35°C the pair cannot be used unless additives are used to move the crystallisation boundary. 

In summary, heat-driven cycles for cooling or heat pumping can have energy saving and environmental benefits. There are also niche applications in developing countries or remote areas. Adsorption cycles using active carbons are one of a number of approaches that might be economically viable. 

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In Chapter 12, one unique application of adsorption for energy utilization purposes is introduced. For refrigeration, cooling and heat pumping application of adsorption phenomena has been attempted. Fundamental ideas on these application are discussed. 

Whilst the above analysis is detailed and quite complex, there are general trends that become apparent relating to how both the carbon properties and the operating conditions affect the OOP s of adsorption heat pumps and refrigerators. The cooling available from the cycle is approximately proportional to the difference between the high and low concentrations and to the latent heat of the refrigerant. The heat input to the cycle has three components the sensible... 

Turner, L., Improvement of activated charcoal-ammonia adsorption heat pumping/refrigeration cycles. Investigation of porosity and heat/mass transfer characteristics. Ph.D. Thesis, University of Warwick, UK, 1992. 

The applieation of aetivated earbons in adsorption heat pumps and refrigerators is diseussed in Chapter 10. Sueh arrangements offer the potential for inereased efficiency because they utilize a primary fuel source for heat, rather than use electrieity, which must first be generated and transmitted to a device to provide mechanical energy. The basic adsorption cycle is analyzed and reviewed, and the ehoiee of refrigerant-adsorbent pairs discussed. Potential improvements in eost effeetiveness are detailed, including the use of improved adsorbent carbons, advanced cycles, and improved heat transfer in the granular adsorbent earbon beds. 

The variation of refrigeration COP with heat rejection temperature (final bed adsorption temperature and condensing temperature are assumed equal) and the maximum cycle temperature is illustrated for an evaporating temperature of -5°C in Fig. 8. Heat pump COP s follow similar trends but are higher. 

L. Turner, Improvement of Activated Charcoal Ammonia Adsorption Heat Pumping/Refrigeration Cycles-Investigation of Porosity and Heat/Mass... 

There are two major types of VOCs recovery refrigerated condensation (Stone 1997) and adsorption, followed by refrigeration (Hussey and Gupta 1997, Kent 1999). Although there are several types of condensation systems the most common adopted is a reverse Rankine cycle where a closed-cycle heat pump with a separate working fluid is used to condense VOCs. Adsorption is a process whereby the VOCs in the air stream are captured physically on the surface of a solid such as carbon. Steam or inert gas is... 

The cooling cycle starts when all parts of the refrigerator are at about 1.3K. At this temperature, the 3He is completely adsorbed by the pump. The pump temperature is now raised to about 25 K by means of an heater. At 25 K, the 3He is desorbed, and its pressure increases over the saturation pressure at 1.3 K. Consequently, 3He condenses in the part of the tube T internal to the copper support C and drops down into the evaporator E. In this phase, the latent heat of condensation and the enthalpy variation are delivered to the 4He bath. The cooling phase starts when all the 3He is condensed in E and the power on the pump heater is switched off. The pump starts cooling towards the bath temperature, reducing the pressure on liquid 3He in E. The adsorption heat of the 3He vapour is delivered to the 4He bath by L. 

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