Pedro Horta1*, Joao Farinha Mendes1, Agustfn M. Delgado-Torres2

1 INETI — Instituto Nacional de Engenharia Tecnologia e Inovagao, IP, Departamento de Energias Renovaveis, Estrada do Pago do Lumiar, 22, 1649-038 Lisboa, PORTUGAL Tel.: +351 217 127 099, Fax.: +351 217 127 195

2 Departamento Fisica Fundamental y Experimental, Electronica y Sistemas, Facultad de Fisica, Universidad de La Laguna, Avda. Astrofisico Fco. Sanchez s/n, 38206 La Laguna, Tenerife, SPAIN Tel.: +34 92 231 8102, Fax.: +34 92 231 8228 * Corresponding Author, pedro. horta@ineti. pt

Abstract

Project POWERSOL [1] aims at the “development of an environmentally friendly improved- cost shaft power generation technology, based on solar thermal energy, optimized for supplying basic need to rural communities”, bringing solar thermal mechanical power/electricity generation to a smaller scale (power outputs up to 500 kW) and allowing the use of cheaper and simpler optical systems, suitable for applications with lower requirements of maintenance and operation expertise, as those found in industry, desalination or small communities.

In the present article, a performance analysis of such a power system is presented, allowing a preliminary assessment of its design. The paper addresses the use of different solar collector technologies in the production of heat, in a range of top cycle temperatures between 150°C and 250°C, suitable for both stationary and single-axis tracking solar collectors.

The use of heat storage and backup systems, allowing extended cycle operation, is also assessed. System performance is evaluated after yearly operation simulation for different locations, enabling an evaluation of the most suitable design for different locations and operation conditions.

Keywords: solar thermal energy; small scale power generation; solar organic Rankine cycle

1. Introduction

The production of electricity or mechanical energy by means of solar energy conversion, has gathered, in the most recent years, a research effort proportional to the increasing interest of such technologies, as a solution covering the needs of either small and isolated communities or large scale power plants.

Whereas the photovoltaic conversion of solar radiation is a well established technology for electricity production, the technologies based in the thermal conversion present a growing number of applications, especially large scale plants based in different types of high concentration systems.

Nowadays, solar thermal driven mechanical energy production is mainly focused in large scale steam solar power plants. Yet, at the present, the use of the steam cycle in low power units or for top cycle temperatures under 350°C is not feasible, for either operational or economical reasons. In

such cases, the Organic Rankine Cycle (ORC) can be an alternative, given its relative high thermal efficiency and the use of dry fluids, impeding vapour condensation at turbine outlet [1-6].

Within the framework of the EU funded project POWERSOL [1,6], a solar thermal driven ORC is being considered, aiming the development of a small scale mechanical power generation technology. The mechanical energy produced could be converted into electricity and/or be used in desalination, water pumping or cooling. The heat rejected in the cycle could also be used — improving the exergy efficiency of the whole system — for water and space heating or cooling.

The core of the power system is an Organic Rankine Cycle (ORC), whose application in power generation at low temperature is well known, with a number of waste heat, geothermal and biomass based examples. In relation to solar thermal — driven ORC’s, there is a lower number of experiences, being the most representatives the Coolidge Solar Irrigation project [2] and the Small Communities Project [3] both developed at the beginning of the 80’s and the recently 1-MW solar thermal power plant operated by the Arizona Public Service in Saguaro (E. E.U. U.) [4]. Angelino et al.[5] also give a number of low power output solar heated ORC systems.

Three different top temperature levels are considered in the POWERSOL project, corresponding to different solar thermal collector technologies: cycle 1 — 80°C (flat plate collectors, FPC); cycle 2 — 150°C (compound parabolic collectors, CPC); and cycle 3 -200°C-250°C (parabolic trough collector and linear Fresnel concentrators). Solar collector prototypes are being developed specifically for each temperature level and a small ORC pilot plant will be erected in the PSA [7].

Considering a preliminary dimensioning of the overall system components (solar field, heat storage, ORC), results for yearly performance of both cycles 2 and 3 are presented in this paper, for three different locations: Almeria (Spain), Cairo (Egypt), Moura (Portugal), considering a system of 100 kW gross power output with different heat storage capacities, operating with or without a conventional energy backup.