Performance characterisation and design considerations for a domestic ammonia/salt resorption heat pump

Heat pumps will play a key role in the future provision of low carbon domestic heating and the re-use of industrial waste heat. Adsorption cycle heat pumps are advantageous in that they can use the existing natural gas network to avoid electricity supply limitations across the UK. A 2 kW domestic-scale ammonia/salt heat pump demonstrator is currently being tested at the University of Warwick as a replacement for a conventional condensing gas boiler. This paper describes analysis work in support of this testing which will lead to design refinements in follow-on developments.

A Matlab-based 2D transient simulation package was developed to study heat transfer and reaction rate within a pair of linked reactors. Heat conduction and sorption rate are modelled together with inter-reactor gas flows and parasitic heat loss. Novel features include the use of Matlab's linked ODE solvers for convergence (ODE15S was found to be fastest) and the script file input configuration which combines clear visibility of parameters with the ability to run multiple simulations to show the effect of parametric variations. The code facilitates rapid design optimisation.

Eleven cycle parameters have been investigated, including filling pressure, heat transfer coefficients, salt ratio, source temperatures, void space and heat capacity. The choice of cycle period involves a compromise between coefficient of performance and power output. A water/glycol heat transfer fluid gives better COP and output power than thermal oil. Insulation within the reactor shell has the potential to limit shell transient heat exchange but void space effects are likely to be more significant. The heat capacity of fluid in pipes and manifolds should be minimised.

COP = 1.31 is achieved at 45 °C delivery; 60 °C for hot water is possible but with lower COP. The best results for space heating are obtained with source temperatures above -5 °C.