Figure 1: Predicted comparative operating costs based on 2010 prices


GLOSSARY ABSORPTION OR ADSORPTION?


Both absorption and adsorption are used to produce refrigeration and to power heat pumps. Liquid absorption systems use


an absorbent (such as lithium bromide) to absorb (dissolve) another fluid (for example, water vapour) to produce a low partial pressure in a connected (cold) evaporator. The water is driven off the absorbent solution using heat after having been pumped into a higher pressure (hot) generator. Then, after cooling and condensing, it is passed back as a low pressure liquid to the evaporator. (www. cibsejournal.com/cpd/2009-11) Adsorption systems use materials with large accessible surface areas (in simple terms a massive ‘sponge’) that will selectively attach, or bond, molecules of working fluid to it by chemical attraction. The huge surface area of cool active charcoal provides an excellent adsorber for organic materials that are subsequently released when the active carbon is heated. This drives the ‘refrigeration’ process.


Comparison of heating costs (UK) 4


3.5 3


2.6


2.5 2


1.5 1


0.5 0


35 Delivery temperature (0 C)


energy supply is from waste heat, the two bed system may be the preferred selection. In Figure 2, performance towards the top


right of the graph indicates both high COP and a smaller amount of absorbent – an overall higher performance. The four-bed performance is superior to that of the thermal wave for COPs less than 1.2 (SCP about 450 W/kg) and gives way to the two bed system for COPs less than 0.6 (SCP about 1500 W/kg). Larger numbers of beds were not considered, since the mechanical complexity of valves and pumps were thought to be excessive. Similar comparisons can be made for


different operating conditions and heating rather than cooling, but the conclusion was reached that a four-bed cycle was most suitable for a domestic heat pump system. It still requires complex valves and extra pumps; but for the conditions suitable for domestic applications, the COP is about 25% higher than that of a two-bed design. To make the system compact enough


for use as a domestic appliance, the team designed a novel shell and micro-tube adsorber unit that has a low thermal mass and yet equivalent heat transfer to a more traditional flat configuration. The adsorber is the core of each of the four


50


n Sorption energy n Standard heat pump n Condensing boiler


2.3 2.1 3.3 3.4 3.3 100C ambient temperature


90% condensing boiler efficiency (HCV)


British Gas gas and electricity prices June 2010


Standard heat pump. Dimplex LI 8 MS


Electric power Heat pump technology has sights on retrofit market


Electrically powered heat pumps can deliver excellent winter performance when supplying an under-floor heating system at 35C. Existing UK radiator heating systems (designed for 60C to 80C heating water) present a challenge where operating at this lower temperature can lead to a 50% drop in heating capacity. Two main approaches appear to


have reached commercial reality as an air-source heat pump capable of direct retrofit, namely the cascade cycle and the economised vapour injection cycle. Most major suppliers now have cascade or split units that typically utilise R410a in their lower stage (outdoor unit) and R134a in their higher stage (indoor unit). Performance of these units produces coefficients of performance (COPs) at more than 4.2 at European test standard (EN14511) and about 3 at higher


32 CIBSE Journal August 2011


temperatures. The University of Ulster’s research has developed the economised vapour injection (EVI) cycle in conjunction with Emerson/ Copeland Ltd. Initial laboratory results were promising and a unit was field trialled in a 105m2 semi- detached house giving a seasonal COP of 3.7. However, a number of challenges arose over part-load operation in summer, using a single heat pump to meet the whole heat demand (marginally oversized for winter conditions) suffering failures due to very short cycling in hot-water-only mode; variable speed drives have been investigated to help overcome this. Numerous companies,


particularly in Asia, have developed EVI heat pumps for higher temperature delivery air source heat pumps. Other research by Ulster in cooperation with EA


Technical Services is developing a combined compressor expander device to recover energy from the higher temperatures and pressures associated with heat pump retrofit applications. A compact unit was developed


to recover power from a turbine. The initial unit designed for R134a has a mass flow 0.0018 kg/s for a compressor electrical demand of 3 kW at 3000 revs/min. It is designed for a maximum operating compressor delivery pressure of 15.4 bar absolute, leading to a heat output of 15 kW and is based on the ability to link a hinging vane with a piston. This technology enables recovery of energy from low-grade heat and efficient expansion and compression of air or refrigerants. Compression and expansion ratios can be varied on demand over a wide range of speeds. Initial results were promising but deteriorated


over time as internal refrigerant leakages and heat transfer between turbine and compressor negated expander gains. A new unit has been developed with improved design, limiting heat transfer and eliminating turbine/compressor refrigerant leakage – providing superior compression and expansion efficiency. Heat pump technology is


evolving, making it increasingly possible to deliver higher temperatures at good coefficients of performance and reduced capital cost. If linked with demand-side management, benefits accrue for both home owners and electricity utilities that may encourage wide- scale deployment and delivery of economies of scale in manufacture.


By Professor Neil J Hewitt, director, Centre for Sustainable Technologies,University of Ulster


www.cibsejournal.com


Running cost (p per kWh)


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