Conceptual design and optimization of a small-scale dual power-desalination system based on the Stirling prime-mover

Highlights

• A systematic approach for the design of Stirling-desalinations was presented.

• The problem was classified as a multi-objective and multi-criteria problem.

• Various configurations of humidification-dehumidification were examined.

• The foremost system was found for higher productions and lower costs.

• The system was a reliable option for the small-scale power-water production.

Abstract

Producing the electric power using discrete systems as well as providing fresh water is essential for rural areas near to seawater that have no access to electricity and sweet water. In this regards, a small-scale production of fresh water and electricity was considered here. Due to the capability to be fueled with different sources of energy, Stirling engines are a suitable alternative for the discrete production of the electricity for rural areas. Accordingly, a conceptual design of a small-scale dual power-desalination system was presented here. Three configurations of small-scale desalination systems using a humidification-dehumidification system, HDH, was examined to be coupled to the GPU-3 Stirling engine and the waste thermal energy of this engine was delivered into the HDH system. The best operating parameters of the combined systems were found in a multi-objective optimization with the goal of maximization of the generated electricity and fresh water as well as minimizing the product cost, simultaneously. Among three optimized configurations, the best one was introduced using analytical hierarchy process, AHP. The proposed system could deliver 2.58 kW of the electric power as well as 23.3 m³ of the fresh water per day with a production cost of 0.25 $ kWh⁻¹ and 0.66 $ m⁻³, respectively.

Introduction

There is increasing demand for fresh water due to drastically grow in population nowadays. On the other hand, the resources of fresh water are limited, especially in countries with dry weather condition like Persian Gulf region. In these areas that have access to seawater, desalination is an alternative for the production of the fresh water through the use of thermal or electrical energies. In this regard, various types of the desalination system have been invented and employed. Dual productions of fresh water and electrical power through thermal power plants are an attractive alternative; because these desalination systems employ the wasted thermal energy of the power plant. The large-scale production of water and electricity through the thermal power plant needs piping networks and electricity grid to transfer water and electricity to consumers. The large-scale production of water and electricity is not a recommended alternative for regions with scattered populations and rural areas. In this condition, the small-scale production of water and electricity through the use of discrete power generation methods and inexpensive type of desalination is an optimistic alternative. The small-scale electricity in discrete systems is mostly produced by internal combustion, IC, engines. Stirling engine, SE, is another alternative for small-scale power production. This type of engine has a higher efficiency than the IC engine. On the other hand, as SE is an external combustion engine that can be heated up using various types of thermal energy from natural gas to biomasses or animal wastes; therefore, SE is a more desirable option in rural areas compared to IC engines.

The main objective of this paper is to provide a conceptual design for a small-scale production of water and electricity via a dual power-desalination system that works based on an SE prime mover and a humidification-dehumidification, HDH, system.

Regarding the combination of power systems and thermal desalination, several studies have been conducted for large-scale and small-scale production of water and electricity. As an example of a large-scale system, Ansari et al. performed thermoeconomic optimization of the combination of a 1000 MW pressurized nuclear reactor, PWR, power plant and a multi-effect desalination, MED, with thermos-vapor compressor, TVC [1]. In another paper, they performed multi-objective optimization of the same PWR-MED plant with objectives of thermoeconomics as well as the exergetic efficiency [2]. As the bypassing the steam for desalination, reduces the capacity of power generation of the power plant, in a most-recent research, Lee et al. [3] evaluated a large PWR-desalination power plant by using the supercritical S-CO2 Brayton technology with two configurations of the S-CO2 cycle. Numerical analysis of the combination of the desalination system and a solar chimney power plant was studied by Ming et al. [4]. A thermodynamic analysis of a combined power, refrigeration, and desalination system was performed by Sadeghi et al.[5], and a new triple-production system with a number of advantages was suggested. In another work, the coupling between a desalination system and a thermal vapor compression refrigeration plant as well as a Rankine power plant was studied by Ortega-Delgado et al. [6]. Moreover, utilizing the waste heat of a refinery for power and fresh water generation was studied by Sharaf Eldean et al. [7]. In an innovative method, simultaneous production of power and fresh water was introduced through the usage of super-capacitive microbial cells [8]. Finally, a pathway for the synchronize generation of water and energy for a long-term resource planning was introduced by Khan et al. [9]. Various aspects of desalination systems and also their hybrid forms were reviewed in a number of review articles. In this regard, economic feasibility and standardization of cost determinants were reviewed in [10]. In addition, solar desalinations were reviewed in [11], [12], [13], [14], [15]. In addition, combined power desalination systems using the combination of reverse osmosis desalination powered by photovoltaic and solar Rankine cycle were reviewed in [16]. Employment of general forms of renewable energy, including solar, wind, geothermal, and ocean energies in desalination technology were reviewed in [17], [18], [19]. Due to the strong theoretical background for the combined power-desalination plant; nowadays, a number of practical plants such as Fujairah power and desalination plant [20] and Ras Laffan 1025 MW Combined-Cycle Plant [21].

Small-scale cogeneration of the freshwater and electricity is usually performed by employing engines, i.e. internal combustion, IC, engine or gas-turbine, GT, for power generation and thermal energy from the exhaust gases as the heat source of the desalination unit. Salimi and Amidpour [22] proposed a combination of an MED desalination and IC engine for 26.8 m³ day⁻¹ productions of fresh water with the price of 7 $ m⁻³. An experimental investigation of a 1 kWe Stirling engine that was coupled to a single effect thermal desalination system was performed by Cioccolanti et al. [23] to produce 150 L of the fresh water per day. They evaluated the performance of the system under two modes, including batch operation and continues operation modes. They found that the productivities of two modes were 1.13 L kWh⁻¹ and 1.16 L kWh⁻¹ for batch and continuous modes, respectively. In addition, the potential of employing the rejected heat from the cold chamber of solar Stirling engines for water distillation was studied by Al-Dafaie et al. [24] and showed that their proposed system can deliver a feasible amount of fresh water and the electric power as well.

On the other side of research, the humidification-dehumidification system known as the HDH desalination was invented as an efficient and economical method of small-scale freshwater production [25]. Exergetic analysis of a combined HDH and reverse osmosis desalination was studied by Al-Sulaiman et al. [26] and found that for increasing the exergetic efficiency of the system, it is required to reduce exergy destruction in the dehumidifier. For improving GOR of HDH various improvements such as the use of heat and mass transfer augmentation device [27], [28], the usage of a desiccant wheel [29], and employing the multi-stage process [30], [31] were proposed. As an example of a multi-stage process, an experimental evaluation of a two-stage solar HDH system for production of the fresh water was performed by Zamen et al. [32]. In another research, the performance of a single extracted HDH system was compared with HDH system with no extraction, and it was found that the limit for the GOR of the HDH system with no extraction is 3.5 while for the system with a single extraction, this limit is 14 [33]. In these enhancements, a part of air or water streams from one stream was extracted and injected into another stream; therefore, the thermal energy of the extracted stream is recovered in another one. In a novel system for generation of power, refrigeration, and freshwater, Sadeghi et al. [34] performed a thermodynamic analysis of their proposed system and provided optimization of that system. The optimized system could deliver 57 kW of power as well as 91.25 kW of refrigeration effect using an ejector refrigerator. The recent freshwater augmentation techniques in solar stills and HDH desalinations were reviewed in detail in [35].

In this paper, integration of an external heated Stirling engine with three configurations of HDH systems was investigated when the exhaust gas from the combustion chamber of the engine is used as a source of thermal energy of HDH system. These configurations include close air-open water-water heated (CAOW-WH), multi-effect close air-open water-water heated (MECAOW-WH), and close air-open water-air heated (CAOW-AH). In MECAOW-WH system, the air extractions from the humidifier are injected into the dehumidifier. For modeling of the Stirling prime mover, a numerical second-order model called as the Simple analysis [36] was performed. This model was used to predict output power and required heat input to the engine under various operating parameters. Details regarding various types of thermal models for Stirling engines including first-order, second-order, and third-order models were cited in [37], [38], [39], [40]. As operating parameters of the dual Stirling-HDH system, including operating parameters of the engine, were considered as decision variables of optimization, the thermal model was required to be able to evaluate the generated power as well as the delivered heat by the engine to the HDH section. In addition, detail thermodynamic model for the three configurations of the HDH section was performed. Selecting proper operating variables of both Stirling engine and HDH system, each configuration was optimized in a multi-objective optimization process while the target of optimization was maximizing the products of the combined system (fresh water and power) and minimizing the cost of products. The prime mover of the cogeneration plant was the GPU-3 engine a famous prototype of Stirling engine developed by NASA-Lewis. For each configuration within the Stirling-HDH system, a Pareto optimal frontier in three objective space was obtained and three optimal solutions among solutions of the Pareto were selected using LINMAP [41], [42] (Linear Programming Techniques for Multidimensional Analysis of Preferences), TOPSIS [41], [42] (Technique for Order Preference by Similarity to the Ideal Solution), and Bellman-Zadeh fuzzy [43] decision-making tools. This procedure was repeated for three configurations of the Stirling-HDH system. Eventually, the foremost Stirling-HDH system was selected among nine systems using analytical hierarchy process, AHP [44], [45], a famous decision-making tool. This model enabled us to select the most optimistic alternative among nine systems based on the three criteria, including the cost of products and magnitudes of generated power and fresh water. In this method, it is possible to employ these criteria with different weight factors. These weight factors are defined based on the judgments of experts on the field of the proposed system. On the other hand, the AHP decision-making methods can integrate different judgments of various experts in unique weight factors. Using the AHP method and judgments of experts regarding importance and weight of criteria, a unique weight number for each alternative among nine options was found and an alternative with the highest weight factor was chosen as the foremost Stirling-HDH system.

In summary, the contribution of this paper can be summarized as follows:

• Comprehensive thermal models for Stirling prime mover as well as the HDH sub-system were presented. This enables to consider operating parameters of the dual Stirling-HDH systems as design variables of optimization.

• Various configurations of HDH desalination for integrating to the Stirling prime mover were examined for small-scale production of the freshwater and electricity.

• The best design of the dual Stirling-HDH system was presented by multi-objective optimization and multi-criteria decision-making tool for a higher rate of production and a lower cost of production.

• The proposed system could be used to deliver the fresh water and electricity of rural areas that have no access to the electric grid and fresh water.

• A methodology for designing a desalination system based on multi-objective and multi-criteria tools was presented that can any extended to any other capacity, scale, prime mover, and types of desalination.