Optimizing limited solar roof access by exergy analysis of solar thermal, photovoltaic, and hybrid photovoltaic thermal systems

doi:10.1016/j.apenergy.2014.01.041

Applied Energy

Volume 120, 1 May 2014, Pages 115-124

https://doi.org/10.1016/j.apenergy.2014.01.041 Get rights and content

Highlights

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Rigorous theoretical exergy model developed to compare solar energy systems.
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Compared photovoltaic solar thermal hybrid (PVT) systems.
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Also side by side photovoltaic and thermal (PV + T) systems.
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Also photovoltaic (PV) systems and solar thermal (T) systems.
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PVT systems are superior in exergy performance in representative climates.

Abstract

An exergy analysis was performed to compare a conventional (1) two panel photovoltaic solar thermal hybrid (PVT x2) system, (2) side by side photovoltaic and thermal (PV + T) system, (3) two module photovoltaic (PV) system and (4) a two panel solar thermal (T x2) system with identical absorber areas to determine the superior technical solar energy systems for applications with a limited roof area. Three locations, Detroit, Denver and Phoenix, were simulated due to their differences in average monthly temperature and solar flux. The exergy analysis results show that PVT systems outperform the PV + T systems by 69% for all the locations, produce between 6.5% and 8.4% more exergy when matched against the purely PV systems and created 4 times as much exergy as the pure solar thermal system. The results clearly show that PVT systems, which are able to utilize all of the thermal and electrical energy generated, are superior in exergy performance to either PV + T or PV only systems. These results are discussed and future work is outlined to further geographically optimize PVT systems.

Graphical abstract

Introduction

Fossil fuels cannot indefinitely sustain the energy needs of the earth’s growing human population due not only to finite supplies, but also the adverse effects of anthropogenic greenhouse gas emissions on global climate [1], [2]. It is therefore necessary to look for alternative renewable forms of energy [3], [4], [5], [6] such as solar energy, which have previously been shown to be a sustainable solution to society’s energy needs [7], [8]. Currently there are two common systems that utilize the sun’s energy for human use: (1) the solar photovoltaic (PV) cell, which converts sunlight directly into electricity and (2) the solar thermal (T) collector, which converts solar energy into thermal energy. As the levelized cost of PV has dropped quickly [9] to become competitive with conventional grid electricity in specific regions, available roof top space with open solar access tends to drop precipitously in those same regions as they are covered with PV. Thus, when attempting to meet all of a building’s internal electricity and heat loads with energy from the sun, roof area becomes a significant limiting factor [10]. A hybrid solar system, called a solar photovoltaic thermal hybrid system (PVT), provides a potential solution to this challenge [11], [12], [13]. PVT systems exploit the heat generated from the PV system, which is normally wasted, to produce useful thermal energy along with the electricity from the PV.

There have been several methods to compare PVT systems using economics, carbon dioxide emissions, energy produced and exergy efficiency [14], [15], [16], [17], [18]. Both Erdil et al. [19] and Kalogirou et al. [20] calculated the economic feasibility of a PVT system and concluded that their systems were cost effective. However, economic analysis is usually used to determine the cost viability of the system, but is limited because of the arbitrary nature of the current economic system [21], [22]. The proposal of using carbon dioxide (CO₂) emissions, particularly the dynamic life-cycle emissions [4], as a way to rate energy systems is useful particularly in the context of stabilizing global CO₂ concentrations. However, trying to make a system more energy efficient would reduce the CO₂ emissions of the system in a given location, which eventually reduces the complexities of varying geographic emission intensities due to fuel mix in a region [4]. Energy analysis has shown that PVT systems produce more energy than either a PV or thermal collector system per unit area [23]. Through this work, studies have tested using different flow rates, glazes and designs to determine if PVT systems are superior [24], [25], [26], [27]. However, like the other two comparisons, energy lacks the ability to compare electrical energy and thermal energy since energy analysis only looks at the quantity of the energy and not the quality as well. Exergy, defined as the maximum useful energy in a specific reference state, typically the surroundings, analyzes both the quantity and quality. This further allows for an improved analysis and optimization of systems since exergy, unlike energy, is not conserved, but rather destroyed by irreversibilities in real processes [28].

There have been several studies comparing PV, T and PVT systems using exergy. However in these studies, the exergy analysis uses a simplified model by multiplying the Carnot cycle by the thermal energy efficiency [29], [30]. Other exergy analysis work has focused on specific systems to try to optimize operating settings [31], [32], [33]. A meticulous exergy analysis comparing PV, thermal and PVT systems has not been undertaken. Thus, this paper provides a more rigorous theoretical exergy model by building on previous detailed exergy models [31], [32], [33] but going further to compare a conventional two panels PVT (PVT x2) system to a side-by-side (PV + T) system, two modules PV (PV x2) only system, and a two panels T (T x2) only system to determine the technically superior system for applications with limited roof area. In this study all four solar energy systems were analyzed for the same total area to ensure an unbiased comparison in three locations with varying climatic conditions: Detroit, Denver and Phoenix.

Section snippets

Nomenclature

Table 1 contains the nomenclature for the equations in Section 3 Material and methods, 4 PV model, 5 Solar thermal model, 6 PVT model. The equations used in these appendices are from the renowned account of solar engineering of thermal processes [34] unless otherwise stated.

Material and methods

Models, detailed in the sections following, of the four solar energy systems (PVT x2, PV + T, PV x2, and T x2) shown in Fig. 1, were created and analyzed in Scilab, an open-source numerical simulation tool [35]. The National Renewable Energy Laboratory National Solar Radiation Data Base 1991–2005 update Typical Meteorological Year 3 (TMY 3) data was used for the three locations: Detroit City Airport (725375), Denver Intl AP (725650) and Phoenix Sky Harbor Intl AP (722780) [36].

All three locations

Solar photovoltaic cell model

In this simulation, the solar PV cells are modeled with a five-parameter equivalent electric circuit which describes the cell as a diode [39], [40]. The starting equation for the model of the solar cell describes the solar cell as a diode and can be seen in Eq. (1). $I = I_{L} - I_{D} - \frac{V + {IR}_{s}}{R_{sh}} = I_{L} - I_{o} [e^{\frac{V + {IR}_{s}}{a}} - 1] - \frac{V + {IR}_{s}}{R_{sh}}$ where I is the current, I_L is the leakage current, I_o is the reverse saturation current, V is the voltage, R_s is the series resistance and a is the modified ideality factor. A circuit depiction

Thermal design

The solar thermal system was modeled using the Duffie and Beckman equations for a tube and sheet system [34]. The solar thermal model is modeled with eight 4 cm tubes running under the absorber. The coolant used is air with a flow rate for the system of 0.056 kg/s (100 kg/h m²), which was chosen based off the ASHRAE standards for testing of solar air collectors [47]. This flow rate was chosen to be in the middle of the range of the ASHRAE flow rates of 0.01–0.03 m³/s m². The inlet temperature is

PVT model

The PVT model was designed as a single panel air heater with the PV module directly on the absorber plate. The thermal equations can be found in Sections 5.1 Thermal design, 6.2 Thermal component model of PVT [34], [45], [46]. The total exergy of the PVT model is the sum of the exergy of the PV module and thermal system of the PVT system.

Results and discussion

The simulations were run for the PV + T (side by side), PV x2, T x2 and PVT x2 systems of equal area for Detroit, Denver and Phoenix. Fig. 3, Fig. 4, Fig. 5, Fig. 6 show a 24-h exergy efficiency for specific days: the spring equinox, summer solstice, autumn equinox and winter solstice for each location and system combination respectively.

From Fig. 3, Fig. 4, Fig. 5, Fig. 6, it was seen that the PVT x2 system outperforms both the PV + T, PV x2 and the T x2 systems. However, at first and last light

Future work

The primary limitation of this work is the assumption that all the exergy is used for the simulated systems. In solar thermal systems in particular this may not always be a valid assumption. Thus, it is left for future work to refine this base case to take into account primarily thermal loads, but also depending on electrical grid conditions, the electric loads. Such electrical grid conditions could include the use of battery storage either in the form of traditional batteries or utilizing the

Conclusions

This study found that for solar energy collecting systems with identical absorber areas, PVT hybrid systems surpassed the exergy efficiency of both PV + T (side by side) and purely PV systems in three representative regions in the U.S. The PVT system outperformed the PV + T system by 69% and the T x2 system by almost 400% in all the locations. Similarly, the PVT system performed 6.5%, 7.2% and 8.4% better than the PV only system for the Detroit, Denver and Phoenix locations respectively. It is

Acknowledgements

This research was supported by the Natural Sciences and Engineering Research Council of Canada and the Solar Buildings Research Network. The authors would like to acknowledge helpful discussions with S.J. Harrison.