1. Introduction
Efforts to increase energy efficiency have intensified over recent years due to the rapid increase of fossil fuel prices and also the need to decrease harmful emissions to the atmosphere [
1]. Cogeneration allows the combination of various technologies to improve the fuel efficiency of electricity-only power plants or combined-heat-and-power (CHP) systems [
2]. Earlier systems have included combined cycle power plants at large scale (10–100 MWe). In the resulting systems there exists a high level of complexity due to the increased number of parameters. Therefore, there is a need to apply advanced methodologies able to determine optimum solutions. However, this procedure becomes rigorous in case a number of parameters (e.g., thermodynamic, economic and environmental criteria) must be included [
3]. Fuel cell technology has been proposed at the kW to the MW scale in a number of proposed systems. In lower temperature proton exchange membrane fuel cells (PEMFCs), CHP systems have been primarily applied at the kW scale, for smaller residential applications, where low-grade heat (recovered from the fuel cell exhaust) is usually adequate to cover residential load profiles, such as space heating and domestic hot water preparation [
4,
5]. These systems are sometimes operated jointly with vapor compression heat pumps to boost heat generation and/or to provide space cooling [
6]. Such systems have been proposed for single-family households and in some cases for multi-unit residential applications [
7,
8]. Coupling of SOFCs with absorption chiller-heater units has also been proposed for larger scale, decentralized applications such as commercial buildings [
9]. Although the resulting operational configurations have led to high system efficiencies (80% to 90%), their complexity resulted in high capital cost, which often dominates lifecycle cost.
Recent progress in SOFC technology includes important advances, such as higher lifetime, lower capital cost, higher electrical efficiency and simpler fuel processing requirements (in the case of natural gas-fueled systems—as compared to PEMFC technology) [
10,
11]. For small-scale residential applications, SOFC-based, natural gas-fueled micro-CHP systems have been proposed through thermoeconomic modeling and optimization techniques and improved operational strategies [
12,
13]. The application of effective optimization techniques, such as decomposition strategies, have been applied in some cases for the design/synthesis optimization of such systems [
7,
8]. For large-scale applications, the possibility of combining SOFC technology with heat engines, such as gas turbine cycles, has been thoroughly investigated since the early 2000s. In natural gas-fueled hybrid systems, where high temperature SOFC stacks have been integrated with gas turbine cycles, effort was placed on the increase of system efficiency to lower fuel consumption [
14]. Due to the complexity of the proposed systems, the design/synthesis options are usually evaluated with advanced optimization techniques [
15,
16].
More recent research effort has focused on the possibility of combining fuel cell technology with renewable energy sources (RES). The combination of RES with fuel cell technology is a more environmentally friendly solution than decentralized hybrid photovoltaic (PV)-gas turbine systems, because in the latter case emissions are generated on-site, i.e., near the serviced buildings [
17,
18]. The deployment of PV units continues to increase because of significant cost reductions in addition to supportive policies, such as net-metering [
19]. In such systems, excess generation of electricity from RES, e.g., via solar PV panels or wind turbines, can be converted to hydrogen through an electrolyzer unit [
20], stored in a hydrogen storage tank, and then reconverted to electricity when renewable energy is unavailable [
21]. The design of such systems for variable load has proven difficult and in most cases the proposed systems have considered grid-connected operation to allow import/export of electricity, while in other cases a constant load operation was followed [
22,
23]. A combination of RES with natural gas (or biogas)-fueled fuel cell units could allow a rapid deployment of these hybrid systems [
24]. Currently the application of hybrid PV-SOFC systems seems more attractive for commercial buildings as the load demand closely matches the solar energy availability. The use of dynamic or quasi-steady state modeling is usually required to model the system as realistically as possible [
25,
26].
The objective of this research study is the thermoeconomic modeling of a decentralized, hybrid PV-SOFC system for application to a commercial building. The PV subsystem, the fuel cell stack, and the steam methane reformer (SMR) reactor components are modeled in detail to allow a realistic representation of their operation at both design and off-design conditions. In addition, a significant shortcoming of previous studies on hybrid RES-fuel cell systems is the fact that, in most cases, actual load profiles have not been considered. The omission of an actual load profile prohibits the extraction of realistic outcomes on the actual viability of such systems. The current study considers both solar/weather data and an actual load profile for a commercial building for the whole year. This approach leads to a more accurate determination of the thermoeconomic characteristics of the proposed system, allowing a direct comparison to conventional useful energy generation. The fuel processor (pre-reformer) is of the SMR type, since it is more efficient than other technologies (e.g., partial oxidation), allowing more efficient natural gas conversion to hydrogen [
11]. The current research study investigates the economic competitiveness of the proposed system in comparison to conventional or alternative power generation. Four different cases are investigated and compared, namely: (A) Central power grid connection (conventional), (B) central power grid connection assisted with PV arrays, (C) non-grid connected SOFC system and (D) decentralized hybrid PV-SOFC system (proposed system). The outcomes of the research work are expected to reveal the possibility of combining and utilizing two highly advantageous technologies, i.e., PVs and solid oxide fuel cells, with an analysis beyond theoretical predictions. This is done with a detailed thermoeconomic modeling of the components, and further on with their overall integration in the system model. Moreover, through the development of a cost model, a complete thermoeconomic analysis is facilitated to lay out the characteristics of the proposed hybrid system.
5. Conclusions
In this study a decentralized, hybrid PV-SOFC system is proposed for the fulfillment of a load profile for a commercial building (small hotel) in Cyprus. The system components are modeled in detail to allow a realistic simulation of the operation of the system. An actual load profile and solar/weather data are fed to the system model to determine the thermoeconomic characteristics of the proposed system. The system is sized based on the requirements of the load profile, with maximum power outputs for the PV and SOFC subsystems at 70 and 152 kWe, respectively. The system operates efficiently throughout the whole year for a transient load profile. The average net electrical and total efficiencies for the SOFC subsystem are 0.303 and 0.700, respectively. Maximum net electrical and total efficiencies reach up to 0.375 and 0.756, respectively. The total contribution of the two subsystems on a yearly basis for the fulfillment of the load profile is at 135.9 and 451.2 MWh for the PV and the SOFC subsystems, respectively. Application of the proposed hybrid system is favored over conventional power generation with electricity-only central power stations for technical and economic reasons. The proposed system can operate more efficiently in terms of net electrical efficiency (especially at part-load operation over a heat engine-based power generator), and, more importantly, it can take advantage of the heat recovery capability of the SOFC subsystem. Additionally, fuel consumption is reduced significantly, primarily because of the integration of the PV subsystem, and also due to the elimination of transmission and distribution losses.
The cost analysis of the proposed system shows that in terms of capital cost, the highest cost is for the purchase of the SOFC subsystem (607,540 USD), while the cost for the PV arrays is 140,132 USD. The cost of the power subsystem, which is usually underestimated, is also significant at 221,951 USD. The total cost of fuel for the operation of the system during its lifetime is estimated at 891,735 USD. Although natural gas prices are constantly fluctuating, it is not expected that this cost estimation will be significantly altered in the near future for the EU market, based on a statistical review of the prices for the last 10 years [
33]. The lifecycle cost for the system is 1,241,369 USD, with a unit cost of electricity at 0.1057 USD/kWh. The proposed system outperforms conventional and other system configurations, in terms of both the unit cost of electricity and CO
2 emissions. In comparison to the conventional case, the unit cost of electricity is about 50% lower (0.2128 vs. 0.1057 USD/kWh), while the reduction in CO
2 emissions is about 36% (673 vs. 428 g(CO
2)/kWh). The additional capital cost for purchasing the PV and the SOFC subsystems is well reasoned by the reduction in fuel consumption, hence on the unit cost of electricity. Similarly, in terms of CO
2 emissions, the proposed system manages to significantly reduce emissions, because power generation is independent of the inefficient central power grid supply. Additionally, the integration of the PV subsystem allows a significant reduction in power generation from the SOFC subsystem during solar energy availability.