Ahmed Hamza H. Ali1, P. Noeres2 ,C. Pollerberg2* and C. Doetsch2

1 Department of Mechanical Engineering, Faculty of Engineering,

Assiut University, Assiut 71516, Egypt

2 Fraunhofer Institute for Environmental, Safety and Energy Technology UMSICHT,

Osterfelder Strasse 3, 46047 Oberhausen, Germany

* Corresponding Author, clemens. pollerberg@umsicht. fraunhofer. de

Abstract

The synchronous occurrence of solar irradiation and cold demand and its causality led to the development of solar cooling facilities. Previously, a number of prototypes and demonstration plants have been erected world wide. Several techniques and concepts are discussed currently and there is still a high necessity for research and development. But there are also first operational experiences gathered by the existing plants. For the further development of solar cooling and the implementation of these systems into the market, it is important to evaluate and to interpret the available experiences.

Since 2002 Fraunhofer UMSICHT has operated a solar cooling plant on its premises in Oberhausen, Germany. The plant target cold capacity is 58 kWth for space cooling. Most of the motive heat energy is provided by a solar tube collectors field while the backup heat source is the heating system of the institute. The plant also provides solar surplus heat to the institute heating system in spring and in autumn. Furthermore, the plant concept allows the operating mode “free cooling”, which means that the cold capacity is exclusively provided by the re­cooling unit when the ambient air temperature is low enough.

The proposed paper describes the solar cooling plant and its integration in an existing energy supply network. In addition, it summarises and analyses the operating experiences of the years 2002 to 2007.

Keywords: Solar cooling, free cooling, operational experiences, absorption chiller

1. Introduction

In the last years, a number of solar cooling plants were erected to investigate the possibility of using a renewable energy sources for cooling applications and to demonstrate their feasibility. The use of the sun as renewable energy source is particular obvious, because solar irradiation and cooling demand is normally synchronized. Since the year 2002 Fraunhofer UMSICHT has operated a solar cooling plant on its premises in Oberhausen, Germany.

Nowadays, about 100 solar cooling air-conditioning systems are installed in Europe according to [1]. Their primary energy savings potential is estimated between 30-60 %. Balaras et al. reported in [2] that a total of 54 solar air-conditioning projects in Europe are in operation, 33 plants are working with lithium bromide-water (LiBr-H2O) absorption chillers. Single-effect lithium bromide-water absorption chillers are the most popular machine in solar cooling due to its ability to operate at low temperature. There are a lot of publications available dealing with the description and a first assessment of respective cooling plant, but long time operational experiences are rarely seen. Ward and Lof [3] described a first integrated system which provides heating and cooling to a building by use of solar

energy. The plant had been installed in a residential building at Colorado State University. It was designed to cover 60 % of the heating and cooling load of the building. Ward et al. reported in [4] for the Solar House I, where a solar heating and cooling system has been operated and monitored in the duration of August 1st, 1974, to January 13th, 1975. The installation provided 40 % of the cooling load by solar energy. Furthermore, Ward et al. reported in [5] about the possibility to improve the average coefficient of performance (COP) of the cooling system by the use of a cold water storage. Bong et al. describes in [6] a solar-powered 7 kWth absorption chiller in Singapore. The system included heat pipe collectors with a total area of 32 m2, an auxiliary heater, a hot water storage tank, and a 17.5 kWth cooling tower. The overall average cooling power provided was 4 kWth, COP of 0.58 and a solar heating fraction of 39 %, which means 60 % of the driving heat work was provided by the auxiliary heater. In [7] Al-Karaghouli et al. reported the operation results of a solar cooling system which was installed at the Solar Energy Research Center in Iraq. The system is equipped with two absorption chillers with a cold capacity of 235 kWth for each, 1577 evacuated tube collectors and 2 thermal storage tanks of 15 m3. They reported that the daily average solar collection efficiency is 0.49, the COP of the chiller is 0.618 and the solar heating fraction is 60.4 %. In [8] Yeung et al. reports about a solar driven absorption chiller with a cold capacity of 4.7 kWth at the University of Hong Kong. The system included flat collectors with a total area of 38.2 m2 and a 2.75 m3 hot water storage tank. The collector efficiency is estimated with 0.375, the annual system efficiency is 0.078 and the average solar fraction is 55 %. Best and Ortega summarized in [9] the operational experiences of a solar cooling plant from 1983 to 1986 in Mexico. The system consists of 316 m2 flat plate collectors, a 30 m3 heat storage tanks, LiBr-H2O absorption chiller with a cold capacity of 90 kWth and a cooling tower with re-cooling power of 200 kWth. The solar heating fraction reaches 53 % to 75 % and the average COP of the chiller is 0.53 to 0.73. In ref. [10] Syed et al. report about the performance of a absorption chiller with 35 kWth nominal cooling power driven by hot water from flat plate collectors with a size of 49 m2 and a 2 m3 hot water storage tank. The designed cold capacity output of the solar cooling plant is only 10 kWth, so that the daily and period average COP reaches only values from 0.34 to 0.42. Pongtornkulpanich et al. described in ref. [11] the installation of a solar driven absorption cooling system with a cold capacity of 35 kWth in Thailand. The system has a 0.4 m3 hot water storage tank and 72 m2 sized evacuated tube solar collectors filed that delivered yearly an average solar heating fraction of 81 %.

In order to summarize the above cited operational experiences, reported solar driven single-effect lithium bromide-water absorption chillers for space cooling applications have a solar heating fraction from 39 to 95 % and reach COP values ranging from 0.34 to 0.73. This work presents operational experiences over a five years period of such a system. The solar cooling plant is located on the premises of the Fraunhofer institute UMSICHT in Oberhausen, Germany, and belongs to the infrastructure. Ali et al. [12] reported the performance assessment of this plant.