Elsevier

Renewable Energy

Volume 48, December 2012, Pages 161-172
Renewable Energy

Performance assessment of a novel system using parabolic trough solar collectors for combined cooling, heating, and power production

https://doi.org/10.1016/j.renene.2012.04.034Get rights and content

Abstract

In this study, performance assessment of a novel system based on parabolic trough solar collectors and an organic Rankine cycle for combined cooling, heating and power (CCHP) is presented. In this system, a portion of the waste heat is used for heating through a heat exchanger and the other portion is used for cooling through a single-effect absorption chiller. This study considers three modes of operation: a solar mode, which is characterized by a low-solar radiation; a solar and storage mode, which is characterized by a high-solar radiation; and a storage mode, which is the operation of the system at night time through a thermal storage tank subsystem. To assess the performance improvement of the present system, three further cases are considered: electrical power, cooling-cogeneration, and heating-cogeneration. This system is designed to produce 500 kW of electricity. Different output parameters – efficiency, net electrical power, and electrical to heating and cooling ratios – are examined in this study. This study reveals that the maximum electrical efficiency for the solar mode is 15%, for the solar and storage mode is 7%, and for the storage mode is 6.5%. Alternatively, when CCHP is used, the efficiency increases significantly. The maximum CCHP efficiency for the solar mode is 94%, for the solar and storage mode is 47%, and for the storage mode is 42%. Furthermore, this study shows that the electrical to cooling ratio is sensitive to the change in the ORC pump inlet temperature. Therefore, the variation in this temperature could be used as a good control for the amount of the cooling power needed.

Highlights

► The energetic performance of four cases considering three modes of operations are compared. ► The three modes are solar (SM), solar and storage (SSM), and storage modes (StM). ► The maximum electrical efficiency for the SM is 15%, for the SSM is 7%, and for the StM is 6.5%. ► The maximum CCHP efficiency for the SM is 94%, for the SSM is 47%, and for the StM is 42%. ► The electrical to cooling ratio is sensitive to the change in the ORC pump inlet temperature.

Introduction

Finding a more energy-efficient system with low or zero greenhouse gas emissions is one of the major concerns of energy producers. One potentially efficient technology is CCHP. It is a type of plant operation in which the production of power, heating, and cooling is based on a single energy source. It is sometimes called trigeneration. In a CCHP plant, the waste heat from the plant prime mover is used to provide heating and cooling energies. That is, a portion of the waste heat from the prime mover is used for heating; for example, it heats water to produce steam. The remaining portion is used for cooling, for instance, cooling air.

Several energy sources can be used to run CCHP plants. Solar energy is considered to be one of the most promising energy sources. It is a free renewable energy source with no gas emissions. The number of power plants operated partially or completely by solar energy has increased considerably recently. The solar energy can be used to obtain electrical power directly through photovoltaic solar cells or indirectly through a solar thermal system.

Different solar thermal systems – such as parabolic trough solar collectors (PTSC), solar towers, and solar dishes – can be used to produce electrical power through thermal power plants. Parabolic trough solar collectors are the most established technology among the thermal solar technologies for power production. They have been used in large power plants since the 1980s. Currently, several thermal solar power plants are under construction and most of them are based on PTSC. Therefore, PTSC is selected in this study.

One of the potential subsystems that can be used in CCHP plants for mechanical power production and, then, electrical power production through an electrical generator is an organic Rankine cycle (ORC). The ORC is characterized by using an organic working fluid; unlike steam Rankine cycle, which uses steam as a working fluid. The ORC can be used when a low- or medium-temperature energy source is available, whereas the steam Rankine cycle can be used efficiently when a high-temperature energy source is available.

Thermal power plants are distinguished by their prime movers. A comprehensive review of CCHP plants based on CCHP prime movers was conducted by Al-Sulaiman et al. [1]. They noted that while a significant number of studies exist on CCHP plants that are based on internal combustion engines as prime movers; only a few are available where gas turbines and microturbines are the prime movers. Same is valid for the other three prime movers: fuel cells, Rankine cycles, and Stirling engines. In terms of analysis type, most of the studies have been conducted using energy and economic analyses. On the other hand, less attention has been given to environmental, exergy, and thermoeconomic analyses of CCHP plants. CCHP plants are usually used in large residential and commercial buildings, chemical power plants, and airports.

Several studies considered using solar energy and ORC for desalination, e.g. [2], [3], [4], [5], [6], [7], [8], [9]. On the other hand, a few studies examined the feasibility of using a thermal system based on solar energy and ORC for electrical power production, e.g. [10], [11], [12], [13]. Jing et al. [10] studied the feasibility of integrating an ORC with a low temperature PTSC at different locations through energy analysis. In their study, they compared the efficiency of single-stage solar collectors with two-stage solar collectors. They found that the two-stage solar collectors have higher efficiency where the improvement varies from 8.1% to 20.9%, as compared to the single-stage solar collector. In addition, they compared the power output of their system at six different locations. In another study, Prabhu [11] studied the feasibility and economics of using PTSC and ORC for electrical power and water cooling production. His study examined the feasibility of using their system in different locations.

Riffat and Zhao [12] assessed the performance of a heating-cogeneration system using solar collectors, an ORC, and a gas boiler. The authors found that the electrical efficiency of their system was 16% and the heating efficiency was 43%, resulting in an overall efficiency of 59%. Zheng and Weng [13] studied the performance of a cooling-cogeneration system for power and refrigeration production using R245fa as a working fluid for the ORC. The refrigeration power is based on an ejector refrigeration cycle. The authors found that the energy efficiency of the system was 34.1% while the exergy efficiency of the system was 56.8%.

In this study, a novel system using PTSC for combined cooling, heating, and power (trigeneration) production is proposed and investigated thermodynamically. The study considers the operation of the solar subsystem in three different modes. The three modes considered in this study are: a low-solar radiation mode, a high-solar radiation mode, and the thermal storage mode (night time). Further details of these three modes are described in the next section. This study is different from most of the studies in the literature that considered only constant solar radiation (one mode of operation) or the variation of the solar radiation during day-time and ignored the need for using power at night and, consequently, did not include a thermal storage system. This study evaluates the performance of the proposed system considering the three modes of operation through key output parameters. These parameters are energy efficiency, net electrical power, and electrical to heating and cooling ratios. The investigation considers the effect of changing different operating variables on these parameters. These variables are the ORC evaporator pinch point temperature, ORC pump inlet temperature, and turbine inlet pressure. The current study assesses the performance of the proposed new CCHP system and quantifies its efficiency. Furthermore, it is extended to examine the performance of three cases – electrical power, cooling-cogeneration (electrical and cooling), and heating-cogeneration (electrical and heating) cases – and compare their performance to the proposed CCHP system.

Section snippets

Description of the CCHP system

The CCHP system considered consists of a PTSC, an ORC, a heating process heat exchanger, and a single-effect absorption chiller, as shown in Fig. 1. This system works as follows: The heat transfer fluid (HTF) in the solar subsystem that is heated through the PTSC is used to heat the fluid in the ORC through evaporator-a and evaporator-b and the waste heat from the ORC is used for heating and cooling. The waste heat from the ORC is used to produce steam in the heating process, using a heat

Modeling

The modeling of the PTSC and thermal storage tanks, and performance assessment equations of the CCHP system considered are presented in this section. The model is programmed using Engineering Equations Solver (EES) software. The input data used in this model is given in Table 1. The performance analysis applied to the single-effect absorption chiller is similar to the approach used by ASHRAE [19] and Herold et al. [20]. The assumptions used in the single-effect absorption chiller are [20]

  • Pure

Results and discussion

In this study, the energy analysis of the CCHP system is conducted based on the modes of operation mentioned above (solar only, solar and storage, and storage only). The discussion of the results in this section is organized as follow. For example, when discussing the energy efficiency, the solar mode is described first, then, the solar and storage mode, and finally the storage mode. In this study, the effects of the ORC evaporator pinch point temperature, pump inlet temperature, and turbine

Conclusions

The performance assessment of the CCHP system using three modes of operation: solar, solar and storage, and storage is conducted. The thermodynamic modeling of this system is performed by varying the ORC evaporator pinch point temperature, ORC pump inlet temperature, and turbine inlet pressure. The main findings of this study are summarized below:

  • The solar mode provides the highest energy efficiencies, and net electrical power. The solar and storage mode has lower energy efficiencies, and lower

Acknowledgment

The authors acknowledge the support of King Fahd University of Petroleum and Minerals (KFUPM), Dhahran, Saudi Arabia, and the Natural Sciences and Engineering Research Council of Canada (NSERC).

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