Elsevier

Renewable Energy

Volume 134, April 2019, Pages 828-836
Renewable Energy

Preliminary study of integrating the vapor compression cycle with concentrated photovoltaic panels for supporting hydrogen production

https://doi.org/10.1016/j.renene.2018.11.087Get rights and content

Highlights

  • The concentrated photovoltaic panel is integrated with the vapor compression cycle.

  • A combined heat and power system is thereby formed to support hydrogen production.

  • A preliminary study comparing the energy availability to requirement is conducted.

  • The compressor power has substantially influenced the system's electric efficiency.

  • The system's selective heat discharge function showed 4% H2 production improvement.

Abstract

Although implementations of CPVs to support electrolysis to produce hydrogen have recently been established, they often neglect the heat component which is important to improve electrolysis efficiency. Therefore, this research proposes to integrate the vapor compression cycle to concentrated photovoltaic panels to form a combined heat and power system that efficiently supplies such required energy to a water electrolyzer to produce hydrogen. The vapor compression cycle is modified to selectively choose whether the waste heat should support water heating or be discharged into the ambient environment where the latter method is aimed to save power consumption. A preliminary investigation of the proposed system is conducted by analyzing a commercial concentrated photovoltaic panel and simple formulations such as the theoretical Carnot efficiency for the vapor compression cycle. Results suggest that the required compressor power in the vapor compression cycle is very substantial and caused the effective electrical efficiency to drop from 40% to 10% for condenser temperature of 360 K–410 K. Fortunately, the heat available by the vapor compression cycle is much higher than that demanded by water electrolysis. Thus, the modified vapor compression cycle with the selective heat discharge functionality has demonstrated hydrogen production rate improvements of up to 4%.

Introduction

Hydrogen is progressively becoming an important green fuel that is projected to replace the nonrenewable fossil fuel and the environmental issues associated with its usage. It is the primary fuel for fuel cells, a device that is already being utilized in electric vehicles [1,2] or as a micro combined heat and power (m-CHP) system [3,4]. Furthermore, although currently less mature than fuel cell technologies, hydrogen combustion engines are also potentially competitive in the future because they readopt the same well-established combustion engine design [5]. Clearly, many recent clean energy technologies are highly dependent on hydrogen as the fuel source and that establishing a global hydrogen fuel economy is of paramount performance to support these systems.

On the other hand, establishing the hydrogen fuel economy is a major challenge because it requires an economically and environmentally feasible method of producing hydrogen. According to a recent report by the US department of energy [6], most fuel cells designed for CHP applications will reform hydrocarbon fuels such as natural gas or biogas into the required hydrogen for the fuel cell. However, such reforming devices suffer from various issues such as high system complexity, requirement of extremely high temperature operation (up to 850–900 °C), emission of CO2 and the contamination of produced hydrogen with CO, CO2 and any unused raw fuel. Alternatively, the recent review [7] conducted a comprehensive survey of various state of the art hydrogen production methods and concluded that thermochemical pyrolysis and gasification methods, which adopt biomass as the raw fuel, are the most economical methods for producing hydrogen. Nevertheless, similar to reforming methods, these methods also require high temperature operation (up to 650 K-800 K), high system complexity and have issues with handling the purity of produced hydrogen.

Electrolysis is an alternative hydrogen production method that utilizes the electrochemical decomposition principle on a material such as water [8] or methanol [9,10]. The former case of water electrolysis is advantageous in that it is a noiseless and pollution free operation. However, the primary challenge behind practicalizing water electrolysis lies in the requirement of supplying electricity to the electrolyzer. Indeed, to justify the usage of electrolysis for supplying the hydrogen economy, the electricity source should not involve hydrogen (e.g. Fuel Cells), does not emit harmful pollutants and shall be renewable – Hence, renewable energy is the most appropriate solution. In this aspect, there are a number of recent research methodologies involving different types of renewable sources for the hydrogen production application [11]. For instance, the usage of geothermal energy for supplying both electricity and heat to a proton exchange membrane (PEM) electrolyzer was proposed in Ref. [12] and showed that amongst a number of refrigerant candidates for the required Organic Rankine cycle (ORC), R245-fa performed the best result with energy and exergy efficiencies reaching 3.511% and 67.58% respectively. A similar study was also conducted in Ref. [13] who concluded that the hydrogen production rate could improve 9 times by increasing the geothermal fluid from 130 °C to 200 °C. Alternatively, a system that integrates solar thermal collectors and photovoltaic (PV) panels to support a high temperature steam electrolyzer was studied [14] with energy efficiencies reported as 9.1% at 873 K and 12.1% at 1273 K.

In the meantime, the concentrated PV (CPV) systems is a class of technology that adopts concentrators to concentrate sunlight into a relatively small PV area. Its key advantage lies in the capability to lower the PV size, which means higher cost PV cells such as multi-junction cells can be more practically utilized. However, due to the high concentration of sunlight, CPV systems typically require an active cooling system to control the cell temperature and maintain a high operating efficiency. In this aspect, the thermoelectric generator (TEG) is considered as a potential cooling option [15,16] where [17] showed that if the TEG's coefficient or performance (ZT) is equal to 1 or higher, the performance of the CPV-TEG hybrid system is significantly higher than the CPV only counterpart. Nevertheless, as established in Refs. [17,18], the introduction of the TEG into the CPV causes the CPV to have a higher temperature and the effective drop in efficiency due to this often negates the TEG's energy harvesting benefits. Alternatively, PV cooling technologies have been reviewed in Refs. [19,20] which covered methods including air and liquid circulation and spraying. Both references concluded that amongst the various studied techniques, liquid circulation techniques are most promising in terms of energy and exergy perspectives as well as the practicality of storing thermal energy in liquid storage tanks.

The adoption of the CPV system for supplying electricity to a water electrolyzer to produce hydrogen has recently been investigated by M. Burhan et al. in Refs. [[21], [22], [23]]. Specifically, an Arduino controlled experimental platform was developed in these researches where the focus in each reference is in the master slave computer hardware arrangement for [21], the tracking system design in Ref. [22] and the integration of these two components in Ref. [23]. The common conclusion from these references is that the sunlight to hydrogen production efficiency using the proposed CPV system is up to 15%–18%. On the other hand, these studies did not consider using water at elevated temperatures, a method that can substantially reduce the electric power required by the electrolysis process. In this aspect [24], investigated three solar system configurations to supply power and heat to a high temperature steam electrolyzer – The first is using solar heat collectors and thermodynamic cycles, the second is using PV panels and electric heaters and the last is integrating solar heat collectors and PV panels together. The study concluded that the integrated solar heat collector and PV panel solution generates the highest sunlight to hydrogen production efficiency of up to 9% but at the price of high system complexity.

Currently, studies on adopting the CPVs as a supplier or both heat and power (hence as a CHP system) exist where, for example [25], experimentally studied the electrical and thermal efficiency characteristics of a water cooled CPV system with waste heat being recovered via the flowing coolant water. Recently, the reuse of PV and CPV based water cooling methods have been highlighted in review reports such as [26,27]. However, the waste heat generated during CPV operation is usually a very significant value and is certainly suitable for increasing the input water temperature that is supplied to the electrolyzer (hence reducing the electricity requirement to produce the same amount of hydrogen). On the other hand, increasing the CPV temperature will also decrease its conversion efficiency and reduces its lifetime. Thus, supplying hot water from a water tank to directly act as the coolant is generally not an acceptable solution, especially when the hot water temperature is up to 80 °C, the optimal temperature required by a PEM electrolyzer [28]. As of the current state of the art, alternative options to directly adopting hot water directly as the coolant include using phase change materials [29] or using a thermoelectric cooler device [30]. However, the practicality of phase change materials is questionable in terms of the ability to supply higher grade heat (relative to the CPV operating temperature). Also, thermoelectric cooler devices have relatively low cooling coefficient of performances (COP) which means they are generally required to consume almost all of the CPV generated power (or even requiring more) in order to support the CPV's cooling requirements. Indeed, state of the art implementations of CPV systems to both act efficiently as a CHP and to support water electrolysis are still immature and require significant design improvements.

Hence, this research proposes to implement the vapor compression cycle (VCC) with the CPV where the purpose is to not only support CHP operation by recycling the CPV generated waste heat into the hot water, but also to maintain the CPV temperature to operate near the ambient temperature (hence increasing its efficiency and lifetime). Moreover, the VCC is innovatively modified to contain two condenser (heat discharging) paths which are either into ambient environment or into stored hot water. The former method is aimed for saving compressor power consumption whenever water heating is not required, and the appropriate condenser path can be selectively decided by a pair of three-way valves. Specifically, the focus of this research is to conduct a preliminary investigation into the feasibility of the proposed CPV-VCC system for hydrogen production by using simple formulations that calculate the power requirements and output characteristics of the individual components. For instance, the VCC's compressor power consumption is evaluated based on the theoretical Carnot efficiency formula multiplied by a system efficiency factor. The influences of temperature on CPV's power output is also included and compared to the consumed VCC compressor power. Most importantly, the CHP output of the CHP-VCC system will be compared to the input requirements of a water electrolyzer, where the electrochemical process is known to consume both power and heat. The aforementioned power characteristics will be studied by using two case studies where the first involves fixing the VCC's evaporator temperature (at the CPV) and varying the condenser temperature. The second case studies the vice versa scenario. Overall, this research provides valuable insight into how the CPV-VCC system should be operated in real time and indeed is indispensable for researchers and designers to continue future work of the proposed design.

The remainder of this paper is structured as follows. Section 2 describes the model structure and equations of the CPV-VCC hybrid system as well as the interface of its output to the electrolysis requirements for producing hydrogen. Section 3 presents the research results by first defining the case study conditions for two cases then presents the corresponding results. Section 4 concludes this paper.

Section snippets

Overall structure

Fig. 1 shows the overall structure CPV-VCC system that will be studied. The CPV panel is the primary power source who is provided concentrated solar power (QIn) by a concentrator (not shown in Fig. 1 for the sake of brevity). Specifically, Fig. 1 (a) shows an illustration of the overall concept and Fig. 1 (b) shows the energy flow diagram of the corresponding system, where the details of the energy flow variables involved are given in this section's subsections. The CPV panel converts a portion

Case study conditions

The imposed fixed conditions for the case studies are provided in Table 2. The primary controllable parameters are then the evaporator and condenser side temperatures of the VCC system (TC and TH). Therefore, two case studies will be considered for study. The first case study will involve the evaporator temperature TC being held at constant ambient temperature (Tamb) whereas the condenser temperature TH shall be parametrically swept. It is noted that typical refrigerants used in VCCs have a

Conclusion

This research has conducted a preliminary investigation into integrating the vapor compression cycle (VCC) to a concentrated photovoltaic (CPV) panel to form an innovative hybrid system that can supply combined heat and power (CHP) to a water electrolyzer for producing hydrogen. A commercial CPV panel which is sized for domestic applications is selected in the study and the VCC performance is analyzed by using the theoretical Carnot efficiency formulation multiplied by a system efficiency

Acknowledgements

This work was partly supported by Key project of national key R&D program for HPC under grant 2016YFB0200603 and Special Program for Applied Research on Super Computation of the NSFC-Guangdong Joint Fund (the third phase) under Grant No. U1501501. The project of Guangzhou Science and Technology program (Grant No. 201704030089) also supports this research.

References (38)

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