R. Salgado*, A. Burguete, M. C. Rodriguez, P. Rodriguez

ITEA Research Group. Universidad Carlos III de Madrid, Departamento de Ingenieria Termica y de Fluidos.
Address: Avda. Universidad 30, 28911, Leganes (Madrid), Spain; Tel: (+34)916248884
Corresponding author: rsalgado@,ing. uc3m. es

Abstract

An important issue of solar cooling facilities based on absorption cycles and sometimes not giving the necessary attention is the recooling process of the absorber and condenser. This is critical in the overall behaviour of the facility because the condensation and absorption temperatures will affect the COP and cooling capacity of the chiller. Most of the time, the recooling process is made by using a wet cooling tower in a closed loop through the absorber and condenser. The use of a wet cooling tower gives good results in terms of cooling capacity and COP, but presents some health risk, like legionella, and its use is restricted to the industrial sector and places where water scarcity is not present. This paper presents the modification of the already validated TRNSYS simulation of a solar cooling facility, implementing a geothermal heat sink instead of the wet cooling tower in order to dissipate the heat generated internally in the absorption chiller. Simulation results shows that a geothermal heat sink composed of 6 boreholes of 100 meters of depth should be sufficient in order to substitute the wet cooling tower, for a typical Spanish single family dwelling.

Keywords: Solar cooling, geothermal heat sink, TRNSYS simulation, Experimental solar plant

1. Introduction

Solar thermal cooling facilities based on absorption cycles, when applied to the domestic sector are being extensively investigated these days because of their great potential on lowering the overload on electricity grids during summer season, but also for their energy and environmental advantages. Most commercially available water fired absorption chillers use the BrLi-H2O working solution. This solution is widely known and its performance has been well documented by many researchers [1, 2]. These types of chillers commonly need wet cooling towers for recooling, in order to dissipate the heat generated internally in the absorber and condenser, thus limiting their use in both the industrial and residential sector for its cost and bulk but also posing the health risk of legionella. Also the operating cost of a wet cooling tower gets incremented by the need of running the cooling fan, consuming a considerable amount of electric energy, and for replenish the water that gets evaporated and dragged out of the cooling tower. These drawbacks are very impeding for the domestic sector.

There are some commercially available absorption chillers recooled by an air stream, even being fired by hot water [3]. In hot days the ambient temperature result too high for them; as a result their COP and cooling capacity substantially diminishes. This effect is more pronounced when the machine is driven by the limited temperature of hot water produced in solar collectors.

An option for absorption cycles recooled by water is the use of a geothermal heat exchanger, taking advantage of the lower soil temperature. The installation of this kind of heat sink is more complex than installing a wet cooling tower because of the previous excavation of the borehole heat exchangers (BHE). The properties and type of soil underneath will dictate the dimensioning and hence the total cost of the facility, but once installed, its maintenance cost is low. Making use of this type of heat sink could promote the use of these facilities in the more sensitive domestic sector.

The Universidad Carlos III de Madrid (UC3M) counts with an experimental solar thermal cooling facility using a wet cooling tower coupled to an absorption chiller for heat rejection purposes. A numerical simulation using the TRNSYS tool has been accomplished and validated over the current experimental set-up.

This paper presents the modification of the already validated TRNSYS simulation, implementing a geothermal heat sink instead of the wet cooling tower. Different BHE connected in parallel and different borehole depths are analyzed in a trial an error way in order to select the best configuration for supplying the necessary heat rejection rate of the absorption chiller.