300
Indirect solar cylinders
Frequently in winter the balance of energy
250
required to satisfy the demand is supplied by
the primary heating appliance – a heating boiler
200
or direct-fired water heater. During the summer
period, the solar energy absorbed by the collectors
and transferred into the hot water can negate
150
Athens
the need for any energy at all being provided by
Birmingham
the primary heating appliance – this can have a
100 Zürich
significant impact on reducing carbon dioxide
emissions and reducing energy bills.
For example, consider the system in
50
Figure 4, a direct-fired water heater system to
raise the incoming cold water supply at 10
O
C
0
to a legionella-safe water temperature of 60
O
C
Jan Feb Mar Apr May June July Aug Sept Oct Nov Dec (ie a separate boiler is being used for the space
heating). A pre-heat cylinder served by an array
Figure 1: Monthly solar irradiance on a flat plane facing south with a tilt angle of 45° for
of roof-mounted solar collectors can be used
Athens, Birmingham and Zurich (Data from BS EN 15316-4-3:2007 Part 4-3: Heat generation
to supply pre-heated feed water to the direct-
systems, thermal solar systems)
fired storage water heater, so requiring less fuel
> operational issues. In the summer months, an cent. An installation of flat plate collectors is to raise the water to the required set point of
appropriately designed solar thermal system shown in Figure 2. 60
O
C.
should be able to satisfy almost all of the hot The construction of an evacuated tube In the summer months there may be
water demand in many cases. collector is entirely different to that of a glazed sufficient solar irradiation over prolonged
The SF is much lower during the colder flat plate collector, although materials used are periods of the day, such that the water in the pre-
winter months, when the available solar common to both types – copper tubes to carry heat cylinder is able to reach temperatures in the
irradiation is a lot lower, resulting in SF the heat transfer fluid, a copper absorber with region of 75
O
C to 80
O
C. In such circumstances,
levels of around 20 per cent. To increase the a selective coating, and tubes manufactured depending on how the pre-heat cylinder and
annual average SF in the winter months, a from glass with a low thermal expansion collector array have been selected, the solar
larger number of solar collectors would be coefficient.
needed – but the array would be oversized Evacuated tube collectors generally comprise
for the summer period and this could lead to a manifold and a series of glass tubes (20 or
stagnation resulting in possible long-term, 30) connected in parallel. A vacuum is created
irreparable damage to the solar collectors. within each tube during the manufacturing
process; this effectively acts as an insulator for
Principal types of solar collector the absorber and reduces convection losses,
There are two main types of solar thermal particularly during colder winter periods.
collectors currently being used in the UK While transmission efficiencies, absorption
commercial building services sector. These are efficiencies and emissions are comparable to
glazed flat plate and evacuated tube collectors. those offered by glazed flat plate collectors, the Figure 2: Application of roof-mounted flat plate solar
The typical construction of a glazed flat plate thermal efficiency is higher as a result of the
collectors – each panel has a 2.55m
2
gross area, 2.21m
2
collector comprises a lightweight aluminium presence of the vacuum, with values of around
absorber surface area, 90.8 per cent transmission
efficiency and 95 per cent absorption efficiency
tray or frame, which contains a layer of insulation 83 per cent. An installation of evacuated tubes
to prevent heat loss via conduction through the collectors is shown in Figure 3.
rear of the collector. A series of copper pipes Whether glazed flat plate or evacuated tube
is laid in a ‘harp’ or ‘serpentine’ arrangement collectors are used, the optimum angle of
within the insulation, to carry the heat transfer orientation is south facing and the optimum
fluid through the collector. A very thin copper angle of inclination between 30 and 45 degrees
absorber is ultrasonically welded to the copper from the horizontal. Direct flow evacuated
pipes. The absorber has a selective coating in tube collectors, where the heat transfer fluid
order to maximise solar irradiation absorption. is pumped through each tube effectively
Finally, the collector has a transparent connected in parallel, offer greater flexibility
glass cover with a low thermal expansion with regard to the positioning of the collector
coefficient such as borosilicate glass (Pyrex), array. These collectors can be placed flat on the
and high transmission efficiency to minimise roof or vertically on a façade, giving the ability Figure 3: Evacuated tube solar collectors installed on
convection losses. Transmission efficiencies to rotate each tube to optimise orientation and
a mixed-use university building on the south coast.
for good-quality products are more than inclination by about +/- 25 degrees. So, even if
The gross area of each collector (each with 30 tubes)
is 4.25m
2
(3m
2
absorber surface area). This collector
90 per cent, absorption efficiencies 95 per the location is not directly facing due south, the
array heats an indirect solar cylinder with capacity of
cent, emissions (losses) five per cent, and tubes can be adjusted accordingly to maximise
1,500 litres used to pre-heat water for two direct-fired
maximum thermal efficiencies around 78 per solar energy absorbtion. water heaters
74 CIBSE Journal February 2009
www.cibsejournal.com
CIBSEfeb09pp73-76 CPD pages•.indd 74 5/2/09 14:40:42
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