A novel methodology to compare between side-by-side photovoltaics and thermal collectors against hybrid photovoltaic thermal collectors

Highlights

• System-level comparison is introduced instead of component-level.

• CCHP-electricity and Solar-electricity selling prices criterion is introduced.

• Comparison between side-by-side PVs and SCs against PV/Ts in CCHP systems.

• Utilizing PV/T is more efficient and becomes more profitable at max selling price.

Abstract

Hybrid photovoltaic/thermal collectors (PV/Ts) have evolved as a new technology that can intervene with trigeneration systems to form a polygeneration system. Accordingly, many studies have compared the performance of side-by-side Photovoltaics (PVs) and Solar collectors (SCs) against PV/Ts based on several criteria. However, these comparisons between both configurations depended on the performance of these components as individual components not on a system basis. A methodology of real system-level comparison is presented in contrary to component-level comparisons that are available in the open literature. This methodology depends on comparing an optimized Solar-CCHP system side-by-side PVs and SCs, against a PVT-CCHP system with (PV/Ts) instead under a constrained area. The comparison is under the constraints of maximizing a formulated combined efficiency that combines energy, economy, environment and exergy aspects. Results showed that the PVT-CCHP system has higher combined efficiency but with lower NPV. Another novel contribution for determining the actual selling price of both sold CCHP-electricity and Solar-electricity is presented. It’s used for carrying out a sensitivity analysis on the selling price. Results showed that by increasing the selling price, the PVT-CCHP system achieved higher combined efficiency and NPV. These results assured the importance of comparing energy systems based on the system-comparison methodology as this methodology guarantees more improvement in system performance after allowing the configuration, sizing and scheduling of the original CCHP system to change after the solar intervention. Moreover, they came up with the conclusion that using PV/Ts instead of side-by-side PVs and SCs will yield higher combined efficiency but with lower NPV at proposed price mode but with increasing the selling prices of sold electricity, PV/Ts are favorable due to higher combined efficiency and NPV.

Introduction

Trigeneration energy systems have gathered considerable attention from energy specialists and energy economists. When adequately designed, trigeneration systems reduce the overall cost of energy production, and lower the carbon footprint for every energy unit generated. However, their implementation in the buildings’ industry faced many obstacles such as the inefficient sizing of their capacities, and their sub-optimal operational scheduling, which have led to high investment and operational costs compared to conventional systems.

Active solar utilization is from the key parameters for solving energy problems to save resources and environment in Egypt. Despite its advantages, solar energy remains a small fraction of the world’s total energy supply (below 2%) [1]. On a global scale, Egypt is one of the most appropriate regions for exploiting solar energy both for electricity generation and thermal heating applications. In 1991, the solar atlas for Egypt was issued indicating that the country enjoys between 2900 and 3200 h of sunshine annually, with annual direct normal intensity energy yield of 1970–3200 kWh/m² and a total annual radiation intensity energy yield varying between 2000 and 3200 kWh/m² from the north to the south of Egypt [2]. Egypt has achieved a growth rate of renewable energy utilization of 15.8% since 2006 with an increase of 3.4% in 2017 only. This has been associated with a 13.7% increase in natural gas consumption while the electricity generation itself has been increased by 2.9%, leading to an increase in emissions by 2.7% [3].

Based on the above, prior research studied the intervention of solar energy components into CCHP systems to form polygeneration systems. Several publications proposed a methodology for comparing hybrid photovoltaic/thermal to side-by-side PV and solar thermal collectors. Delisle and Kummert [4] submitted a detailed review about comparison methodologies of PV/T that include combined energy or exergy efficiency, combined primary energy saving efficiency, equivalent area or economic factors. The equivalent area was used by Bakker et al. [5] to compare a PV/T water heating system combined with a geothermal heat pump to side-by-side PV modules and solar thermal collectors producing the same amount of thermal and electrical energy. They found that the initial cost of the PV/T system was similar to that of the PV module and solar thermal collector side-by-side, but that the side-by-side system required 32% additional roof surface area. Da Silva and Fernandes [6] used the same methodology to compare a glazed sheet-and-tube PV/T liquid collector with pc-Si modules to side-by-side PV and solar thermal collectors producing the same amount of energy. They obtained annual thermal and electrical efficiencies of 15% and 9%, respectively for a four-person household in Lisbon. Side-by-side PV and thermal systems producing the same amount of thermal and electrical energy would have required an additional area of 60% compared to the PV/T collector.

Delisle and Kummert [4] stated that PV/T technologies aren’t known as their advantages aren’t clear due to lack of comparison methodology. They presented a case study on how this novel approach can be used to demonstrate the actual energy and economic benefits of BIPV/T air systems compared to side-by-side PV modules and solar thermal collectors for residential applications. In this methodology, the thermal energy produced by both systems is transferred into water using a heat exchanger and the concept of annual equivalent useful thermal energy production is used to combine thermal and electrical energy. A case study was performed by simulating the performance of both systems on a 40 m² south-facing roof located in Montreal, Canada. The total energy produced by both systems was assessed by converting electricity into heat with various conversion factors. For a factor of 2, the BIPV/T system was found to produce 5–29% more equivalent useful thermal energy than the PV + T system for a water temperature at the heat exchanger inlet corresponding to 10 °C.

Good et al. [7] have chosen to use the available area as the boundary condition, but the comparison was based on the calculated import/export energy balance. Three alternative solar energy systems for a single-family building were studied through energy performance simulations. The studied systems were a combination of solar thermal and PV (building A), uncovered PV/T or a combination of covered PV/T and PV (building B), and a system with only PV (building C). The systems are based on commercially available modules and standardized templates. The objective of the comparative study was to evaluate whether the building with the different installations would reach a net zero energy balance. The results showed that the system that gets closest to reaching net zero energy balance according to this definition was the system with only high-efficiency PV modules (building C*), even though the solar thermal fraction in this case was zero. The second closest was the system with high- performance solar thermal collectors and PV modules (building A*), which had the highest solar thermal fraction. However, the interpretation of the results depends greatly on the criteria used in the net zero energy building (nZEB) definition, what boundary conditions are used, and the choice of a heat pump as auxiliary energy source.

Huide et al. [8] presented simulation models of the solar thermal, photovoltaic and hybrid photovoltaic/thermal systems with validation experiments. Using the validated models, performances of the three solar systems for residential applications were predicted energy comparison between the three solar systems used in Hongkong, Lhasa, Shanghai and Beijing of China, respectively, were also studied. Results showed that, for the urban residential building with limited available installation space, a hybrid photovoltaic/thermal system may have the largest potential for reducing the energy consumption among the solar thermal, photovoltaic and hybrid photovoltaic/thermal systems. And for a rural house with large available area, system with photovoltaic and hybrid photovoltaic/thermal modules can obtain the most net annual electricity output, and the installation area of the hybrid photovoltaic/thermal collectors mainly depended on the hot water load of the building.

Gürlich et al. [9] compared the annual energetic, exergetic and economic performance of a combined (hybrid) photovoltaic-thermal collector system (PV/T) to that of a system with separate solar thermal and photovoltaic collectors. He concluded that the performance of the PV/T system varies strongly with the boundary conditions of the demand profile, climate situation and energy price levels. Results demonstrated that PV/T collectors used for heat (primary DHW) and electricity offer approximately 6–7% higher exergetic efficiency than thermal and PV collectors equally sharing the available surface area. The analysis shows that the economic performance of the trigeneration PV/T system is only better than separate generation if the additional electricity and cooling generate enough benefit to compensate the loss of heat gains. Especially in the Portuguese location of Almada, with high electricity price levels and high enough cooling demand, the application of PV/T in a trigeneration system is recommendable from an exergetic and economic point of view.

Slimani et al. [10] presented a comparison between four solar device configurations: photovoltaic module (PV-I), conventional hybrid solar air collector (PV/T-II), glazed hybrid solar air collector (PV/T-III) and glazed double-pass hybrid solar air collector (PV/T-IV). The numerical results show that the daily average of overall energy efficiency reaches: 29.63%, 51.02%, 69.47% and 74% for the first (PV-I), the second (PV/T-II), the third (PV/T-III) and the fourth (PV/T-IV) configurations respectively.

Kasaeian et al. [11] conducted a critical review of the literature on solar combined heat and power systems (CHP), which includes solar photovoltaic/thermal systems, concentrated photovoltaic/thermal systems, and various combination with different solar collectors and applications. It showed that there are serious gaps in this field, which calls for more research. There are limited studies on the economic and exergy assessments of the solar concentrating CHP systems. The solar collectors for combined CHP were focused on optimizing the performance of the maximum average useful power generation and minimum total heat transfer area, little environment impact analysis was conducted. They suggested careful exergy, economic and environmental analysis on both electronic and thermal performance, especially for large CHP system. Also, they recommended further studies for investigating the hybrids of concentrating collectors with CHP, with considering the economic issues.

As shown in the previous literature, most studies that dealt with the introduction of solar energy components in trigeneration systems showed that energy and emissions savings increased. However, none of them proposed a criterion for comparing the actual performance of this introduction. Many of them compared based on the concept of comparing the added component to another in the same system which will not benefit the whole system. Solar collectors, PVs and PV/Ts intervene with other trigeneration components to provide the load demands efficiently. That means that the comparison should be based on a system basis not based on producing the same energy required in the same system with same existing capacities. This optimal system-comparison gives a complete picture on the real effect of adding any solar component to a trigeneration system because it gives the solar system the freedom for more intervention with the system resulting in more enhanced performance. At the end of the day, an efficient component in a nonefficient system isn’t the objective but the main objective is an efficient system that contains efficient components working simultaneously. Using this concept comparison of Solar-CCHP polygeneration system (a trigeneration system with side-by-side photovoltaics and thermal collectors) to a PVT-CCHP polygeneration system (a trigeneration system with PV/T panels) under fixed available roof area is more realistic than comparing only the energies produced or the area used by side-by-side PVs and SCs or PV/T panels.

To simulate the real case, this paper provides several novel contributions as listed below:

• A system-level methodology of comparison between two optimized solar systems. The solar systems are Solar-CCHP system that comprises of PVs and SCs side-by-side along with the ICE as shown in Fig. 2 while the other system comprises of PV/T panels instead of side-by-side PVs and SCs on the same roof area as shown in Fig. 3. This methodology guarantees more improvement in system performance after allowing the configuration, sizing and scheduling of the original CCHP system to change after the solar intervention.

• As mentioned in [11], most of the research papers that either compared the performance of intervention of solar energy components with trigeneration systems or compared between different solar energy devices haven’t dealt with the environmental and exergetic aspects of the whole system. Moreover, they haven’t dealt with optimal planning and scheduling of these systems.

• This paper also provides a methodology to compare such systems based on the energy hub concept under the constraints of maximizing a formulated combined efficiency that contains annualized total cost saving ratio (ATCSR), exergy efficiency (EXEff), fuel saving ratio (FSR) and carbon dioxide reduction ratio (CO2RR) using a weighing factor method. This is made by comparing each indicator to a conventional system in GAMS using the RMINLP solver. Using part load effect and variable capital costs of components to simulate the real case, the tool provides optimal planning, sizing and scheduling of all polygeneration systems.

• Another important contribution of this paper, is that it defines a criterion for determining the minimum selling prices of electricity produced from the CCHP and solar devices by determining the cost of electricity production. It also proposes a profit margin for the investors. It also carries out a sensitivity analysis on the value of the selling price of CCHP and solar electricity by optimizing the systems in three modes: No sell mode, proposed price mode and maximum selling mode.