Hybrid photovoltaic and solar thermal (PVT) systems for solar combined heat and power
File(s)
Author(s)
Guarracino, Ilaria
Type
Thesis
Abstract
Solar is a particularly promising sustainable energy source in terms of its potential
to displace the burning of fossil fuels for heat and power, heating and even cooling,
albeit at a cost. The sun load-factor profile has a close and predictable match to
the daily varying energy demand for heat and electricity, both thermal and electrical,
and thermal storage for periods of low irradiance can be made readily available. In
addition, solar thermal technologies can provide a significant fraction of the hot water
demand in households, as well as space heating and cooling in residential buildings
and for industrial facilities. In fact, solar heating has been proposed as one of the
leading solutions in terms of its potential for greenhouse gas abatement [1]. At the
small scale, photovoltaic systems presently dominate the domestic solar market with
solar to electrical conversion efficiencies of around 15% and at a competitive cost for
the building owner. Solar photovoltaic installations were encouraged in Europe at
the local level with financial support and now constitute a large and mature market
with continuously falling prices. Solar thermal systems are able to make use of a
larger proportion of the solar resource as they convert solar energy into heat with a
higher efficiency than the PV conversion efficiency. Moreover, the low temperature
heat may be used to satisfying the largest portion of the demand for thermal energy
that is currently met by fossil fuels. The development of the solar thermal market
is strongly dependent on the availability of the local irradiance level and on the cost
of the alternative sources of thermal energy. In some countries in Europe the solar
thermal market is quite mature (e.g. Austria), whilst in others, such as in the UK,
solar thermal energy still contributes marginally to the energy mix and solar thermal
systems are not yet cost competitive. Due to the high costs of solar thermal energy
systems, these constitute a relatively small market at present with the potential to
grow substantially in the near future. A competitive solution for energy (heat and power) provision in buildings is the
development of combined solar photovoltaic/thermal (PVT) systems which produce
both electricity and heat simultaneously from the same aperture area. This solution is particularly suited to residential applications in urban areas, where the demand for
electricity is accompanied by a demand for low temperature heat, and space for solar
installations is scarce. Many alternative technologies for PVT integration exist and
PVT units can be coupled with various systems for domestic hot water generation
and/or space heating. At the design stage of a PVT system, decisions have to be made
on the absorber characteristics (consisting of thermal collector and PV laminate), on
the thermal to electrical yield ratio and on the application (industrial or residential
application, stand alone or grid connected). These design parameters influence the
requirements on the fluid temperature and electricity output, and the overall efficiency.
In addition, system control can significantly impact the potential of such systems in
terms of their performance characteristics in different applications.
The aim of this present research effort was to demonstrate the technical and practical
feasibility of a novel, high-efficiency hybrid PVT water system, by considering
an affordable, small-scale, modular unit that can be scaled easily to cater to varying
demand levels. The research investigated the technical issues related to PVT panel
technology, by looking in particular at the optical efficiency of the PV cells, at the
heat transfer from the PV cells to the fluid, and at the integration of such a unit in a
heat and power provision system that attempts to match generation and local demand.
A detailed numerical model was developed that constitutes a tool for testing various
collector and system designs. The model was validated against experimental data. An
experimental apparatus was designed and constructed for the purpose of evaluating the
collector model and for collecting a database of performance data on PVT collectors.
Collector performance data are scarce at the moment due to the relatively small market
size, thus the work constitutes a reference for further development and analysis of
this type of collectors. Steady-state tests and dynamic tests were performed on PVT
collectors and the results were used to develop a reliable model of collector performance
over a wide range of time-varying operating conditions.
The model allowed for assessments of various solar PVT system designs under
different operating conditions and control strategies. Result showed that such systems
may underperform if their operation and design is not designed specifically for the local
weather conditions and user-demand specific requirements. It is envisaged that emissivity control applied to the solar cells should be adopted for
PVT system application, especially if higher operating temperatures are required (e.g.
in combination with thermally driven/heat powered cooling systems). The numerical
model confirms that solar cells a with low emissivity coating can maximise the thermal
energy output of a PVT system. The potential of improved PVT systems is finally
assessed from an economic perspective, in an analysis that considers the potential cost
reduction of PVT systems in relation to alternative technologies used as a benchmark.
to displace the burning of fossil fuels for heat and power, heating and even cooling,
albeit at a cost. The sun load-factor profile has a close and predictable match to
the daily varying energy demand for heat and electricity, both thermal and electrical,
and thermal storage for periods of low irradiance can be made readily available. In
addition, solar thermal technologies can provide a significant fraction of the hot water
demand in households, as well as space heating and cooling in residential buildings
and for industrial facilities. In fact, solar heating has been proposed as one of the
leading solutions in terms of its potential for greenhouse gas abatement [1]. At the
small scale, photovoltaic systems presently dominate the domestic solar market with
solar to electrical conversion efficiencies of around 15% and at a competitive cost for
the building owner. Solar photovoltaic installations were encouraged in Europe at
the local level with financial support and now constitute a large and mature market
with continuously falling prices. Solar thermal systems are able to make use of a
larger proportion of the solar resource as they convert solar energy into heat with a
higher efficiency than the PV conversion efficiency. Moreover, the low temperature
heat may be used to satisfying the largest portion of the demand for thermal energy
that is currently met by fossil fuels. The development of the solar thermal market
is strongly dependent on the availability of the local irradiance level and on the cost
of the alternative sources of thermal energy. In some countries in Europe the solar
thermal market is quite mature (e.g. Austria), whilst in others, such as in the UK,
solar thermal energy still contributes marginally to the energy mix and solar thermal
systems are not yet cost competitive. Due to the high costs of solar thermal energy
systems, these constitute a relatively small market at present with the potential to
grow substantially in the near future. A competitive solution for energy (heat and power) provision in buildings is the
development of combined solar photovoltaic/thermal (PVT) systems which produce
both electricity and heat simultaneously from the same aperture area. This solution is particularly suited to residential applications in urban areas, where the demand for
electricity is accompanied by a demand for low temperature heat, and space for solar
installations is scarce. Many alternative technologies for PVT integration exist and
PVT units can be coupled with various systems for domestic hot water generation
and/or space heating. At the design stage of a PVT system, decisions have to be made
on the absorber characteristics (consisting of thermal collector and PV laminate), on
the thermal to electrical yield ratio and on the application (industrial or residential
application, stand alone or grid connected). These design parameters influence the
requirements on the fluid temperature and electricity output, and the overall efficiency.
In addition, system control can significantly impact the potential of such systems in
terms of their performance characteristics in different applications.
The aim of this present research effort was to demonstrate the technical and practical
feasibility of a novel, high-efficiency hybrid PVT water system, by considering
an affordable, small-scale, modular unit that can be scaled easily to cater to varying
demand levels. The research investigated the technical issues related to PVT panel
technology, by looking in particular at the optical efficiency of the PV cells, at the
heat transfer from the PV cells to the fluid, and at the integration of such a unit in a
heat and power provision system that attempts to match generation and local demand.
A detailed numerical model was developed that constitutes a tool for testing various
collector and system designs. The model was validated against experimental data. An
experimental apparatus was designed and constructed for the purpose of evaluating the
collector model and for collecting a database of performance data on PVT collectors.
Collector performance data are scarce at the moment due to the relatively small market
size, thus the work constitutes a reference for further development and analysis of
this type of collectors. Steady-state tests and dynamic tests were performed on PVT
collectors and the results were used to develop a reliable model of collector performance
over a wide range of time-varying operating conditions.
The model allowed for assessments of various solar PVT system designs under
different operating conditions and control strategies. Result showed that such systems
may underperform if their operation and design is not designed specifically for the local
weather conditions and user-demand specific requirements. It is envisaged that emissivity control applied to the solar cells should be adopted for
PVT system application, especially if higher operating temperatures are required (e.g.
in combination with thermally driven/heat powered cooling systems). The numerical
model confirms that solar cells a with low emissivity coating can maximise the thermal
energy output of a PVT system. The potential of improved PVT systems is finally
assessed from an economic perspective, in an analysis that considers the potential cost
reduction of PVT systems in relation to alternative technologies used as a benchmark.
Version
Open Access
Date Issued
2017-06
Date Awarded
2017-09
Copyright Statement
Attribution NoDerivatives 4.0 International Licence (CC BY-ND)
License URL
Advisor
Markides, Christos
Ekins-Daukes, Nicholas J.
Sponsor
European Commission
Publisher Department
Chemical Engineering
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)