Y. Aldali 1 B. Davison T. Muneer 2 e-mail: t.muneer@napier.ac.uk D. Henderson School of Engineering and Built Environment, Edinburgh Napier University, 10 Colinton Road, Edinburgh EH10 5DT, UK Modeling the Behavior of a 50 MW Direct Steam Generation Plant for Southern Libya Based on the Thermodynamic and Thermophysical Properties of Water Substance This paper presents arguments for the use of direct steam generation (DSG) in preference to other forms of generation in particular locations according to the prevailing environ- mental and economic conditions. In addition, the paper describes the development of a software tool based on Microsoft Excel and Visual Basic for Applications (VBA), which draws upon established physical relationships in the heat transfer literature to perform plant capacity calculations in a fast and convenient manner. The results of the VBA pro- gram determine the solar fraction of the plant, assuming that the plant is in operation for 10 h per day (07:30–17:30 hours), the solar fraction is shown to be 76% and the DSG plant achieves a 76% reduction in emissions. Construction costs are also estimated based on formulae from previous work. [DOI: 10.1115/1.4006893] 1 Introduction The need for alternative approaches to electricity generation is being driven in the 21st century by social, economic, and environ- mental factors. The demand for electrical appliances in develop- ing countries, the fears about the continued availability of oil and natural gas, and the spiraling levels of greenhouse gases in the atmosphere all motivate the development of reliable, clean, and sustainable alternatives. Although several technologies exist for capturing renewable energy from sources such as the wind and the sea, concentrated solar power (CSP) is one of the most attractive options. In contrast to the capture of solar energy in photovoltaic (PV) cells which requires exotic and expensive materials, CSP offers a simpler and cheaper approach. Solar power is concen- trated on a collector, typically using parabolic mirrors, to induce high temperatures which are used to generate steam to drive a tra- ditional turbine. Current commercial CSP plant employs an indi- rect design in which a heat transfer fluid passes through an absorber tube where solar energy is collected, and then through a heat exchanger to generate steam. DSG in the absorber pipe with- out the need for a heat exchanger eliminates the need for the syn- thetic oils typically used as HTF and offers several advantages over the indirect design [1]: • Environmental risks associated with synthetic HTF such as fires and leaks are eliminated. • A higher maximum temperature can be sustained compared to the current limit of around 400  C above which synthetics HTF tends to degrade. • Losses due to the oil/steam heat exchanger are lower leading to greater plant efficiency and lower construction costs. • Plant design is simplified since with the elimination of the heat exchanger and auxiliary thermal oil systems. • The simplified design has lower operation and maintenance costs since an auxiliary heating system for HTF is unneces- sary, and there is no requirement to replace a proportion of the HTF each year. The present authors believe that development of DSG technol- ogy requires an understanding of the two-phase water/steam flow within the system. Similar views have been expressed in Ref. [2]. This paper reports on the development of a software model of the thermodynamic and thermophysical properties of water and steam in a hypothetical 50 MWe CSP plant, which takes into account the environmental conditions in the chosen geographical location. In addition, the model includes land requirement calculations for the plant, and the specification of a secondary natural gas-fuelled boiler required to maintain the design output of the overall facil- ity. For simplicity, this paper does not consider the engineering implications of the balance of plant required for maintaining a steady flow rate around the closed loop circulation system, or for condensing the exhaust steam from the turbine ready for recircula- tion. This constitutes the main limitation on the work presented. 2 Context Following on from an earlier feasibility study for a CSP plant generating less than 1 MW [3], the design context for the current study is a 50 MW plant located at 24  17 0 N, 23  15 0 E at Kufra in the south-eastern part of Libya, an area which forms part of the Sahara desert. This location benefits from a high average clearness index of 0.67 [4] and in addition there is ample supply of water from subterranean aquifers and local production of natural gas. The aim of the model is to describe the thermodynamic and thermophysical properties of the water and steam, which act as 1 On leave from Mechanical Engineering Department, Faculty of Engineering, Omar Al-Mukhtar University, Derna, Libya. 2 Corresponding author. Contributed by the Solar Energy Division of ASME for publication in the JOURNAL OF SOLAR ENERGY ENGINEERING. Manuscript received October 27, 2011; final manuscript received May 2, 2012; published online June 14, 2012. Assoc. Editor: Manuel Romero Alvarez. Journal of Solar Energy Engineering NOVEMBER 2012, Vol. 134 / 041001-1 Copyright V C 2012 by ASME Downloaded 20 Jul 2012 to 216.91.96.130. Redistribution subject to ASME license or copyright; see http://www.asme.org/terms/Terms_Use.cfm