Hong Kong, a modern sub-tropical city, has
a great potential of applying solar energy in hotwater and electricity cogeneration. Most buildings here
are provided with conventional gas/electrical water heaters
for serving hot-water supply. According to the energy end-use data (2010) published
by the Hong Kong Government, direct hot water production
consumes about 5% of the total energy use in the city,
and 20% when considering only the domestic sector. Since 2005, the Hong Kong Government has
been targeting at 2012, with 1-2% of the territory-wide
energy use from renewable energy sources. On the other hand, direct
electricity generation from photovoltaic (PV) is more expensive than conventional
power generation. The situation is expected to remain
for some years.
By integrating PV modules with solar water heating
collectors, the increased energy yield per unit collector
area is able to shorten the economical payback period
to less than 1/4 of the plain PV application. Such photovoltaic/water-heating
(PVW) cogeneration is found to be much promising than the separate
side-by-side installations of PV modules and solar thermal collectors from
energy conversion, space utilization, and aesthetic points of view. The rationale
behind the integration is that a PV cell available in the
commercial market converts solar radiation to electrical
energy with a peak efficiency in the range of 6-15%,
depending on the specific PV-cell type in use. Most of
the incident solarenergy is converted as heat, leading to
an increase in the cell working temperature. By cooling
the PV module with a stream of water, the electricity
yield can be improved. In many places including Hong
Kong, the promoted use of renewable energy has become a long-term government policy. With
more and more solar systems required to be installed in
buildings, the available roof space and facade surface
area will sooner or later become inadequate for the wider application
of solar energy. The hybrid PVW collector thus has a market potential to become a
popular commercial product.
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Dr Tin-tai Chow and the photovoltaic/
water-heating cogeneration system
An escalating number of research activities
on PVW has been reported in the last few years. But so far the literatures were reporting
the numerical or experimental findings, with inadequate coverage on
real plant operation and system reliability. The international
research community has placed real project demonstration with longer-term monitoring as
a current strategic need of the technology.
Accordingly, a PVW system has been
constructed to serve an office building at our Kowloon Tong campus. The warm water
generated is to serve a selected number of pantries as tap-water source for general use.
The electricity generated from the
polycrystalline silicon solar cells is to support the power supply to a
group of EXIT signs of the building. The working experiences gained
from this small-scale PVW system is to provide practical information for
improving the engineering design and installation of future hybrid
PVW systems in buildings. The field measured data will be analyzed
from energy saving, sustainability, reliability and finance aspects, as
compared to the conventional or alternative design options in Hong
Kong.
Dr Tin-tai CHOW
Division of Building Science and Technology
City University of Hong Kong
bsttchow@cityu.edu.hk
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