|
Sunny side up
Solar energy has great potential to reduce carbon emissions but it’s
important to choose the right applications and understand the need for
integration with other heat sources. Ian Dagley of Hoval explains
The current drive to include renewable energy sources in projects has
undoubtedly fuelled a greater interest in the use of solar thermal energy for
heating and hot water services. In the UK, however, solar irradiation levels
fluctuate widely and can range from less than 100Wh/m2 of collection area on
a cloudy day to over 1000Wh/m2 on a sunny day.
Consequently, any solar heating system needs to be backed up by 100%
auxiliary heating, which could be other low carbon heat sources such as
biomass boilers or heat pumps, which will enhance the renewable element of
the project. Or there may be other circumstances where it is more
advantageous to combine solar heating with condensing boilers.
When solar heating is used for domestic hot water (DHW) the system has to
be very flexible and able to respond to rapid changes in conditions. On sunny
days, there will often be sufficient solar energy to heat all of the water to the
required temperature. And when less solar energy is available it may be used
to pre-heat mains cold water before it is brought up to temperature by another
heat source.
To a very great extent the optimum design will depend on the detail of the
project. As a rule of thumb, the significant shifts in demand that are inherent in
larger projects mean it is usually more cost effective to install solar equipment
to pre-heat the DHW, and then top it up to the required temperature.
Not only does this keep the capital cost down, but it also reduces the
frequency that the solar circuit goes into stagnation (periods when no energy
is being removed from the collectors by the solar fluid) during the summer
months. This in turn increases the system efficiency and specific energy
produced by the collector loop.
It’s also important to note that any stored volumes of pre-heated potable water
should be kept to a minimum, as these may have to be regularly pasteurised
as part of the anti-Legionella regime. Pasteurising large volumes of potable
water is not only costly, but it also has a dramatic affect on the solar yields
and subsequent solar fractions attainable. In these circumstances it is
beneficial to store the solar energy in a thermal storage vessel and pre-heat
the cold feed water through a suitable heat exchanger. This also has the
benefit of allowing the solar energy to be safely stored at higher temperatures
without the risk of scalding.
Piping a number of non-potable thermal storage vessels in series and using
diverting valves to circulate the water in the vessels will enhance stratification
within the vessels. This will promote solar gain and also allow biomass boilers
to be fully integrated within the heating/DHW scheme and support the solar
powered system.
During summer months when solar energy is in abundance, another way to
lessen the periods of stagnation is to increase the angle of inclination on the
collectors. While this reduces the efficiency of the collectors in the summer it
increases the efficiency in the winter when the sun is lower in the sky, thus
having a load levelling effect. Consequently, this is an ideal solution for low
grade heating such as underfloor systems and can also contribute to DHW
demand in the summer.
Solar heating is also particularly good for heating swimming pools because
the pool acts as a large heat sink (no need to buy thermal storage vessels),
and the relatively low operating temperatures (typically 26-30°C) are easily
achieved. These low temperatures result in high collector efficiencies and
solar fractions. Additionally most indoor pools have a large roof area, which is
ideal for the solar collectors.
So, when considering renewables such as solar the key is to decide which will
work well together in a controllable fashion and which overall solution gives
the best results for that project. For instance, in some schemes the maximum
benefits may be attained by using solar heating backed by biomass boilers for
DHW and condensing boilers to take advantage of low return temperatures
from the space heating system.
In fact, this illustrates the key consideration for integrating renewable
technologies into any building services project. The first stage is to consider
the technologies that may be suitable for each project and to ensure you
understand the pros and cons of each technology. Then it’s a matter of finding
the best mix of these technologies and control strategies – again in relation to
the specific conditions dictated by each project. This understanding and the
ability to apply sound engineering principles to innovative solutions is where
the skills of building services engineers have a vital role to play in reducing
the UK’s carbon emissions.
Ends |
