Pedro Adao1*, Manuel Collares Pereira1, Andrea Costa2

1 Ao Sol SA (SunCool SA), Lugar da Sesmaria Limpa, 2135-402 Samora Correia, Portugal
2 ACE, Pfalzerstr. 75, 83109 Grosskarolinenfeld, Germany
* Corresponding Author, pedro. adao@aosol. pt

Abstract

This work presents the development results of a novel absorption chiller for solar cooling applications built at Ao Sol (SunCool laboratories) in Portugal. The chiller is at pre­commercial stage at the time of writing. Focus of the ongoing development work is the optimisation of technical and economic parameters such as overall dimensions. A thermo­economic analysis will give insight on the commercial potential and challenges.

Under present conditions, dictated by a new European legislation and current fuel price, the use of small absorption chillers coupled to solar thermal technology for residential applications turns out to be potentially both environmentally sound and economically viable. Necessary requirements are cost-efficient solar collectors and chillers, and an economically optimized combined system.

Relying on previous work performed at the University of Lisbon, a compact absorption chiller working with a solution of ammonia and water has been built and tested. The device is mainly made of plate heat exchangers and it is directly cooled by air, thus avoiding the need of a wet cooling tower. The control strategy is aimed to an overall concept that includes collector field, storage, gas back-up, chiller and building.

The thermodynamic design is presented along with experimental results of the thermal performance — in terms of chilled water production and coefficient of performance. The economic impact of the developed chiller for a common residential application in Portugal is compared with a reference system and discussed.

Keywords: solar cooling, air-cooled chiller, ammonia-water absorption, net present value

1. Introduction

The present European Directive on Energy establishes ambitious goals for the year 2020: 20% contribution of renewable energies to the final energy demand and 20% reduction of the final energy consumption through energy efficiency measures [1]. This creates a very strong drive for the solar penetration in the Heating and Cooling of Buildings, responsible for about 40% of the total final energy consumption in the EU [1].

Combining this directive with the ESTIF solar thermal installed capacity goals for 2020 [2 ESTIF Newsletter July 2007] an upper limit of about 4 million new solar driven machines can be anticipated for the residential and services market.

If the more ambitious ESTIF expectations are taken into consideration this figure will be multiplied by a factor of 5 to 10 to 2030.

The present situation in Europe shows a huge potential for renewable energies. In the building sector, with its highest emissions share, the potential for energy savings is over proportional high. Building cooling is rising to become a crucial issue in the next years. Solar energy is indisputably assessed as a major contributor to these savings and it is being fully backed by policy makers through long-term directives at EU-level.

Trends in Southern Europe: Southern European countries are in a particularly dire situation with a growth of energy consumption in the buildings sector precisely because of the widespread use of electrically-driven air-conditioning units; a situation which will even worsen with the warming trends associated with global warming. In the Portuguese residential and services sector the average increase rate of electricity consumption over the last three years is more than 4 times the average rate of the European Union [3]. This is mainly due to the rise in air-conditioning demand traditionally achieved with highly inefficient small-size chillers (window units). This generates a tendency, which is against the goal of 20% energy use reduction and requires even stronger action than in Northern European countries.

In the past, INETI and some of the people involved in the present development participated in a European funded project [4] to analyze the technical and economic potential of solar absorption cooling in the Mediterranean region. Essentially the conclusion was that — through savings induced by solar energy — an all-year system providing heating, cooling and domestic hot water would compare favourably with a conventional system with a boiler for heating and an electric chiller for cooling with a payback time of less than 10 years. The conclusions and ideas developed in that study were truly influential on the decision of AO SOL to take up the present R&D effort.