divided by the annual reduction in operational CO2 emissions when compared to a fossil-fuelled hot water heating.) Even when the sometimes- optimistic solar utilisation figures used in academic studies are replaced with the average measured data taken from the recent Energy Savings Trust monitoring6 of domestic applications, the carbon performance is still beneficially positive. And, pragmatically, the application of solar thermal hot water can help meet the thermal requirements of the building regulations as part of an holistic design process. Maintenance requirements are relatively
low, and similar to that of a small hydronic heating distribution system. However, as the heat transfer fluid in closed indirect solar thermal systems is protected from freezing by being a propylene glycol solution, there is a greater propensity for leakage compared with plain water. So fill levels need to be checked regularly and the quality of the glycol solution should be checked annually, as it can degrade at high temperatures, reducing its anti-freeze effect as well as increasing its acidity. As from 6 April this year, planning permission is no longer required to install solar PV or solar thermal on non-domestic buildings in England – there are already permitted development rights (PDRs) for domestic installations. This recent amendment to PDRs7
in England is likely
to encourage an increased number of retrofitted installations on commercial and agricultural buildings. The new rules will also mean that ground-mounted systems up to 9m2
will be able to go ahead without
a planning application. There are limiting restrictions associated with the PDR that are mainly associated with the appearance of the installation.
UK government funding The UK government has reaffirmed its support for renewable heating in domestic premises and will set out a firmer timetable for delivering this support in the autumn, with an anticipated introduction of a funding mechanism (likely to be the RHI) from summer 2013. The RHI is already in place for non-domestic applications but has only successfully registered a small number of installations to date. The level of RHI funding (now at 8.9p per kWh of metered heat output) is proving so attractive that some contractors are able to offer installation of solar thermal systems at no cost to the
56 CIBSE Journal May 2012
commercial users, by taking the RHI payments to pay for the scheme. The Renewable Heat Premium Payment (RHPP) ’voucher scheme‘ has been available for domestic applications since the start of this month (May 2012) and provides a £300 voucher (valid for three months from issue) towards the cost of a domestic solar thermal installation when installed by a Microgeneration Certification Scheme (MCS) certified installer (eligibility is subject to the domestic premises having a specified minimum thermal performance). Although £300 may seem a relatively small amount compared to the typical domestic solar thermal cost of around £3,000 to £5,000, the assurance of having the system installed under the auspices of the MCS will ensure that the installations are eligible for support through the RHI, providing they meet the eligibility criteria of any future RHI scheme. There are ring-fenced RHPP funds available for community renewable heating systems, and for social landlords to upgrade heating systems. Again, this is likely to encourage the further application of non-domestic solar thermal systems. The positive life time economics of
solar thermal water heating are currently dependent on the provision of subsidies and would not be likely to ‘pay-back’ in real cost terms when offsetting a fossil fuelled or even an electric hot water heating system.
The ‘inefficiencies’ of operating the solar thermal system The performance of solar thermal is considered in different terms to that of a traditional heating resource. The energy resource itself is practically unlimited (and
Losses Glazing Collector Reflection Conduction convection Re-emittance
40 per cent Solar irradiation 100 per cent 60 per cent 50 per cent
free), and the amount that can be utilised will be related simply to the area that is being captured and the effectiveness of transferring the heat from the incident photons of solar energy to the end use (in this case, hot water). In practical terms, Figure 2 provides an approximate breakdown of where the potential heat is lost. Many of the losses in the yellow and red boxes may be minimised by appropriately insulating the pipework, fittings and storage vessels. (In the EST 20115
survey, poor insulation of
hot-water storage cylinders and pipes contributed significantly to heat loss and low performance, although it notes that this is a common issue with all water- heating systems and not just solar thermal systems.) The typical control mechanism used
by solar thermal systems, the differential temperature controller (DTC), as well as the anti-legionella protection, can have a significant effect on the efficiency of the systems, and so require properly considered design and operation. (Through novel control strategies, other than simple DTC, there are systems claiming that substantial increases of useful heat may be utilised in an annual cycle.) The energy used by the heat transfer fluid circulating pumps will be relatively small5
; however, since they
will be operating at all the times that the solar circuit is in use, they should be appropriately selected to operate at high pumping efficiencies at the design flowrates (the pump may, of course, be powered by solar PV). The solar thermal system will also reduce
the need for an otherwise potentially part- loaded boiler in summer, hence providing a gain in overall system efficiency.
Losses Pump DTC Primary pipes
Losses Storage Secondary pipes
10 per cent
10 per cent Useful energy to taps 40 per cent
Gains Mitigation of boiler inefficiency in summer
Figure 2: A representation of the indicative inefficiencies in a solar thermal system (Source: EST131, 2006)
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