An example of solar technology is adopted to demonstrate the link between sus-tainability and efficiency. An effective way to maintain a good electrical efficiency by removing heat from the solar panels and to have a better overall efficiency of a photovoltaic system is to utilize both the technologies simultaneously. This kind of system is known as hybrid photovoltaic/thermal (PV/T) system and can be beneficial for low-temperature thermal applications like water heating, air heating, agricultural crop drying, solar greenhouses, space heating, etc., along with electricity generation that can further be beneficial for rural electrification and agricultural applications like solar water pumping, etc. In this case study we are giving a simple demonstration on how both the technologies together give better efficiency which directly relates to better sustainability.
Based on the first law of thermodynamics, the energy efficiency of a PV/T system can be defined as a ratio of total energy (electrical and thermal) produced by PV/T system to the total solar energy falling on the photovoltaic surface and can be given as
Here, h , A, T ,,, T I , and V are the convective heat transfer coefficient
' c^ ' cell ' amF s^ oc from photovoltaic cell to ambient, area of the photovoltaic surface, cell temperature, ambient temperature, short circuit current, and open circuit voltage, respectively. The convective (and radiative) heat transfer coefficient from photovoltaic cell to ambient can be calculated by considering wind velocity ( v ), density of the air, and the surrounding (ambient) conditions (Tiwari, 2002).
The exergy efficiency is based on second law of thermodynamics that gives not only a quantitative assessment of energy but also the qualitative. A comparison of PV and PV/T system is also presented in the form of a case study later in this section.
The exergy efficiency of a photovoltaic system can be given as
Fr solar where Ex is the exergy of the PV system which is mainly electrical power output of the system. Since the thermal energy gained by the system during the operation is not desirable in case of PV system, this becomes a heat loss to the system and hence needs to be subtracted from the former in order to calculate the exergy of a PV system. EXsolar is the exergy rate from the solar irradiance in W/m2 which can be given as (Joshi et al., 2008, 2009)
An expression for the exergy of PV can be given as
Here, Im and Vm are the actual current and voltage.
Unlike PV systems, PV/T system uses the thermal energy available on the PV panel and this time the thermal energy gain can be utilized as a useful energy and hence, the exergy of the PV/T system becomes the sum of the electrical ex-ergy and thermal exergy of the system and the exergy efficiency can be defined as
An expression for exergy of the PV/T system can be given as
The relation between exergy efficiency and the sustainability index (SI) can be given as (Rosen et al., 2008)
where Dpis the depletion factor/number defined by Connelly and Koshland (1997) which can be given as
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Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.