CPD PROGRAMME The remaining ‘inefficiencies’, as


shown in the blue box of Figure 2, are related to the physics of the collector. The principal form of the collectors (as discussed in the CPD in February 20098


are ‘flat plate’ and ‘evacuated tubes’, and each has their own performance characteristics and particular attributes. However, the effectiveness of a solar collector may simply be defined by:


Useful heat passed from the collector to the system Incident radiation striking the collector’s absorber


And this relationship has been used to develop the commonly used performance equation for solar collectors – the Hottel- Whillier-Bliss equation9,10 solar irradiance is G (W/m2


, for when the ):


Useful heat = collector area 헑G헑[(FR


where:


휏 is the transmittance of the clear cover; α is the absorber’s shortwave absorptance; FR


is a ‘heat removal factor’ used to allow practically measurable temperatures to be used in the equation; UL


the value of Δθ increases; and (θi


– θa


difference between the inlet heat transfer fluid and the ambient air.


The clear coating (typically glass


or plastic) needs a high value of 휏 at wavelengths associated with solar radiation, as well as providing good insulation to keep UL


low. The absorber surface coating needs


to be ‘selective’, having a high absorbtance in the solar spectrum and a low emissivity in the thermal (infrared) spectrum. Plain metals such as copper and aluminium have low solar absorptance, and so a thin layer of a material with high solar absorptance and good infrared transmittance, such as ‘black chrome’, is applied to the metal . In flat-plate collectors, thermal insulation


is used around the casing to reduce losses. Values of FR


(휏 α) and FR UL are provided


using independent tests (as specified by ASHRAE Standard 93 Methods of Testing to Determine the Thermal Performance of Solar Collectors) by manufacturers as a standardised way of characterising the output of a solar collector. In Europe, BS EN 12975 Thermal solar systems and components — Solar collectors is used as the standard method for defining a collector’s performance. Although based on the same fundamental relationships, the BS EN


www.cibsejournal.com is the overall heat loss factor – this rises as ) (or Δθ) is the specific temperature 헑 휏헑 α)– (FR UL – θa (θi )/G)] 1 )


0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9


0 0K 20K 40K 60K 80K 100K 120K (Mean collector temperature – surrounding air temperature) Figure 3: Efficiency of an example flat plate collector at different solar irradiances


12975 test methods provide three values that are used to define the performance:


Collector efficiency = η0


where: η0


– (a1 x (θm – θa)/G) – a2 x (θm – θa)2 /G) is the efficiency when the collector is at


the same temperature as its surroundings (and is often referred to as the ‘optical efficiency’); a1


and a2 are loss coefficients; and


the ‘collector’ temperature is given in terms of its mean temperature, θm


.


Using data from a manufacturer's catalogue (η0


= 0.779, a1 = 1.07 , a2 =


0.0135), representations of collector performance may be produced, such as that in Figure 3. The characteristics of collectors can vary


widely and deserve appropriate attention (with the same rigour as any other piece of HVAC equipment) when being selected. Properly designed solar thermal systems are a means of capturing the sun’s energy for use in buildings that, until the cost drops or efficiency rises for photovoltaic technology, can provide a most carbon-effective method of generating hot water. To make them most cost effective requires careful selection, design and operation but, unfortunately, without subsidy they are unlikely to break even financially. © Tim Dwyer 2012


Further reading: Solar water heating systems – guidance for professionals, conventional indirect models (CE131) provides a good overview of the technologies and is freely downloadable


from the Energy Savings Trust: www.energysavingtrust.org.uk The CIBSE Solar heating design and installation guide provides some detailed description on systems and appropriate installation details, and the CIBSE Capturing Solar Energy (KS15) (freely downloadable to CIBSE members from www.cibseknowledgeportal.co.uk) gives an accessible and succinct introduction to a number of solar technologies, including solar thermal. BS EN 12975 and ASHRAE 93 provide extensive detail of the testing requirements for solar thermal collectors. The US-based Solar Rating and Certification Corporation – www.solar-rating.org – provides data for numerous tested collectors.


References 1


Solar Thermal Markets in Europe Trends and Market Statistics 2010 – European Solar Thermal Industry Federation, June 2011.


2 Photovoltaic Geographical Information System (PVGIS) – http://re.jrc.ec.europa.eu/pvgis/


3 Croxford, B. and Scott, K., Can PV or Solar Thermal be cost effective ways of reducing CO2


emissions for


residential buildings? Solar 2006: Renewable Energy – Key to Climate Recovery. American Solar Energy Society: Denver, USA.


4 Beccali, G. et al, Life cycle assessment of a solar thermal collector. Ardente, F. Renewable Energy 30 (2005) 1031–1054.


5 Streicher, E. et al, Energy Payback Time – A Key Number for the Assessment of Thermal Solar Systems, EuroSun 2004.


6 Here comes the sun: a field trial of solar water heating systems, Energy Savings Trust 2011.


7 The Town and Country Planning (General Permitted Development) (Amendment) (England) Order 2012 – www.legislation.gov.uk/uksi/2012/748/made


8 Dwyer, T., Solar thermal – solar hot water heating, CIBSE Journal, February 2009 – www.cibsejournal.com/cpd/2009-02/


9 Hottel, H.C. and Whillier, A., Transactions of the Conference on the Use of Solar Energy, vol.2, part 1, Univ. of Arizona Press, 1958.


10 Bliss, R.W., Solar Energy 3, 1959. May 2012 CIBSE Journal


100 W/m2 500 W/m2 750 W/m2 1,000W/m2


57


Collector efficiency


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