Water heating Renewables
> Indeed, the marriage of different technologies
associated with water heating raises a number of
issues, says Evans. “Every day now we see schematics
across our desks containing a mixture of heat sources
in the plant room. But this throws up difficult design
issues. For example, if it is a split system, there will be
boilers and water heaters.
“You might also see combined heat and power
(CHP), maybe a ground source heat pump, and perhaps
a biomass boiler. When you have a mixture of heat
You can’t
sources that have different limitations and different
guarantee that
hot water temperature outputs, you must make sure
solar energy will
you are getting the most out of each individual piece of
technology and that they are not fighting each other.”
be sufficient
That is why the designer must be mindful of the
to keep the
performance benefits and limitations of the different
domestic hot
pieces of equipment, Evans adds. “I think that, as a
consulting engineer, you have to develop a control
water vessel
strategy that allows you to get the best out of each
at 60
0
C
individual technology and therefore the best out of the
– Kevin Stones
system as a whole.”
He offers this example of the design difficulties
associated with a system that includes CHP and solar Eco-Knight non-storage condensing water heater
thermal: “You would size your CHP for the base load;
CHP likes to have both space heating and hot water manufacturer Hoval: “You can use solar energy, but you
loads because, in the summer, when there is no heating would also have, say, a coil in the cylinder linked to a
load to sustain the operation of the CHP unit, it needs boiler or other heat generator to bring the temperature
to have access to the hot water load. up if needs be.”
“If you add a solar thermal system designed to
satisfy summer hot water demand, then you are not Storage solution
putting any load onto the CHP unit. And therein lies Alongside the rising use of renewables such as solar
the problem – this mix of technologies could cause the thermal technology, there has been a significant growth
CHP to cycle on and off – something the CHP engines in hot water storage. Take, for instance, an apartment
don’t like. Equally, the solar system displaces natural block with 150 apartments. Rather than bringing solar
gas, but the environmental benefit of the CHP unit is energy into individual apartments, it might make
not the gas it is displacing; rather, it is the electricity more sense to have a central thermal store feeding the
it is saving. The carbon emissions from electricity are apartments. However, more storage for preheating
higher than those from gas. increases the risk of legionella, warns Stones: “You
“So you are effectively using a renewable to switch can’t guarantee that solar energy will be sufficient to
off a low carbon technology that has more of a carbon keep the contents of a calorifier or domestic hot water
benefit than the renewable does.” vessel at 60
0
C [to prevent legionella], so there is a need
It is fairly common to separate the solar circuit for an anti-legionella pasteurisation cycle.
from the domestic hot water supply and preheat “Some form of back-up heating facility will ensure
the cold water going into the domestic hot water that the temperature of the vessel is lifted to ensure that
cylinder. However, there is an alternative, according any legionella-type bugs that may be present are killed
to Kevin Stones, engineering and services director of off. It’s not ideal because you lose the stratification in >
Renewables: Sources for water heating systems
Solar thermal: There are, according Ground source heat pump (GSHP): This Air source heat pump (ASHP): This takes
to Lochinvar, two basic types of solar is an electrically-powered system that energy from the air and raises it to a higher
thermal collectors – evacuated tube and uses the earth’s relatively constant ground temperature using a process which is
flat plate. While the flat plate has been or groundwater temperature to provide similar to a reverse refrigeration process.
used extensively, especially in domestic heating, cooling and hot water. GSHPs According to one supplier, Danfoss, ASHPs
properties, evacuated tube panels with their move heat that already exists. Results are are ideal for very tight spaces, as well as
higher overall efficiency are better suited lower energy consumption, as well as lower within an eco-architectural design or within
to our northern hemisphere climate. Some maintenance and life cycle costs. However, the design of a building that has large
manufacturers have guides to solar thermal their initial cost can be higher than internal areas such as audience halls and
systems installation and design. conventional systems. public places.
46 CIBSE Journal April 2009
www.cibsejournal.com
CIBSEapr09 pp44,45,46,48_water.indd 46 3/4/09 11:46:40
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