L. Gonzalez1*, E. Rojas1 and E. Zarza2

1 Ciemat — PSA, Avda. Complutense, 22, 28040 Madrid, Spain
3 Ciemat — PSA, Carretera de Senes, s/n, 04200 Tabernas, Almeria, Spain
lourdes. gonzalez@ciemat. es

Abstract

In the last years, it has reappeared the interest that there was in the Eighties of last century in solar power plants with parabolic-trough collectors. These plants allow generating electricity like in conventional power plants, but replacing the primary energy source by the Sun, a renewable energy source. Although the scene has changed from the conditions in the Eighties, now another favourable conditions exist that promote the construction of new systems, both in the United States and in Europe. In particular, in Spain a change in the legislation for electricity generation has encouraged the implementation of renewable energies among which the solar thermal energy is. As the solar field supposes one of the main investments when erecting a thermosolar power plant, it is necessary to have an effective tool for the determination of its dimensions. This paper presents a new methodology for designing optimum parabolic trough solar fields by finding the best value around a first estimation. This first estimation of the solar field is obtained from a simplified calculation of the performance of a parabolic power plant at the site of interest, assuming a fixed solar direct irradiance at the solstice when the Sun gets its maximum declination (summer solstice for northern hemisphere and winter solstice for southern hemisphere). The performance of the power plant along a real year (i. e. using beam solar irradiation and ambient temperature data from a typical meteorological year at that site) and varying the solar field size around the first estimation is analyzed to obtain the optimum solar field. Keywords: thermoelectric, parabolic troughs, solar field, design, optimization, dumping of energy

1. Introduction

The Kyoto Protocol has set the guidelines to be followed to reduce the changes caused by greenhouse gases. This requires promoting energy savings, but above all the use of new sources of cleaner energy (i. e. renewable energies) is necessary. Solar thermal energy is a good example. One of the more promising fields of application for solar thermal energy is electricity generation by means of solar thermal power plants, which have the potential to provide, at least, 5% of global energy demand in 2040, [1].

Solar power plant technology with parabolic trough collectors is one of the more mature technologies at present, [2], with 340 MWe connected to the electricity grid in Southern California (SEGS plants) and 64 MWe in Nevada (Nevada Solar One plant). In 2004, the Spanish Royal Decree 4361 and its reviewed version in 2007 (Royal Decree 661/2007) launched the major Spanish power market players to be among the first 500MW. Most of them are promoting parabolic trough plants with up to 50MW nominal power, like Andasol-1 and 2 in Andalusia or

the Iberdrola (Spanish electric utility company) and IDAE (Spanish Institute for Energy Savings a Diversification) thermosolar power plant in Puertollano, Ciudad Real.

A solar thermal power plant has the same components as a conventional power plant with the exception of the steam boiler, which is replaced by a solar system. This system is mainly composed by the solar field, the heat exchanger and in some cases by a thermal storage. The solar field is formed by a number of parallel rows of parabolic trough collectors connected in series.

The working fluid (thermal oil) circulates through the absorber pipes from the entrance to the exit of each row.

As the solar field supposes one of the main investments when erecting a thermosolar power plant, it is necessary to have an effective tool for the determination of its dimensions. In order to optimize the design of the solar field, the reported simulation tools [3-4] work with an economic criterion as the single figure of merit. It means that every time the specific economical situation to apply changes, the optimization has to be run again from the very beginning — or to leave the design like it was-. Considering the example of Spain, where the premium payment for the electricity produced by solar thermal has been reviewed 3 times in 5 years, an optimization tool based only on economical aspects may waste of lot of design engineers’ time. The optimization methodology presented in this paper establishes an energy-related criterion prior the economic one, limiting to a small amount of new simulations to run if the economical framework changes. The energy-related criterion is aimed at minimizing the waste of energy in summer while maximizing the annual electrical energy production. As the specific feature of the methodology proposed is this energetic optimization, this paper presents the energy-related optimization and not the economical optimization that would follow.

The optimization methodology includes two parts:

* A Pre-design of the solar field. With a simplified calculation process, a first estimation of the solar field size is obtained.

* Optimization itself of the solar field size. The number of collectors per row is kept as in the pre­design, but the number of rows in the solar field is optimized. The number of sizes or, in other words, the different numbers of rows, to simulate is limited by assuming that the waste of energy in summer (called “dumping of energy”) is below 3% of the annual production. The number of simulations is, then reduced to just 3 or 4 cases or sizes. Every simulation gives the thermo­electrical performance of the solar power plant along a year using typical meteorological data at that site. The simulation is based on a simplified physical model of a parabolic trough power plant. A brief example is also presented in the last section to illustrate this methodology.