HEAT PUMPS 1 SORPTION TECHNOLOGY Figure 2: Comparative performance of absorbent bed systems under prescribed temperature conditions 2
1.8 1.6 1.4 1.2 1
0.8 0.6 0.4 0.2 0
0 500 1000 SCP (W kg1 ) 1500 2000 2500 Performance envelopes
2-Bed 4-Bed Modular
regenerated whilst the previously heated beds may be cooled so that they adsorb ammonia vapour again. Critoph considered a large number of
possible cycles, ranging from one known as Shelton’s thermal wave that uses two beds with some simple heat recovery between the two; through variations proposed and developed by Critoph over the last 25 years (some that he has successfully applied to solar-powered vaccine refrigerators); and including isothermal beds with heat recovery as proposed by Meunier. Using modelling developed by the
Warwick team’s Steven Metcalf, they compared the potential two- and four- bed systems as well as a modular system
The team has developed the prototype of an affordable air source sorption heat pump that is small enough to be used domestically
developed using the thermal wave principal to identify the best combination of coefficient of performance (COP) and specific cooling power (SCP in watts per unit mass of adsorbent) for a system used to provide cooling at a prescribed set of temperatures. As shown in Figure 2, whilst the modular
thermal wave can deliver very high cooling COPs, it has a correspondingly very low SCP, which would imply a very large and costly machine (per unit of cooling power). At the other extreme a simple two-bed system (that included heat recovery and mass recovery between beds) has much higher SCPs although lower COPs. If COP was not important, for example when the
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Figure 3: Schematic representation of packaged unit showing the two modes of operation that allow continuous operation
Cool Exhaust Gases
Water to
load Final Exhaust Heat Exchanger
Warm Exhaust Gases
Air-to-Pressurised Water Heat Exchanger
Hot Gases
Absorbent Bed 1
Heated Ammonia Absorbent
Bed 2 Heated
Gas Burner
Ambient Air to Evaporator
Inlet Air
Return water from load
Evaporator
Cooled Air from Evaporator
Condenser
Cool Exhaust Gases
Water to
load Final Exhaust Heat Exchanger
Warm Exhaust Gases
Air-to-Pressurised Water Heat Exchanger
Hot Gases
Absorbent Bed 1
Heated Ammonia Absorbent
Bed 2 Heated
Gas Burner
Ambient Air to Evaporator
Inlet Air
Return water from load
Evaporator
Cooled Air from Evaporator
Condenser
August 2011 CIBSE Journal 31
www.cibsejournal.com
COP
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