SAP 2009 Consultation - the solar thermal aspects

I have complained about the number of consultations that the government creates; most are fairly pointless but once in a while there is one which is important. There is currently underway a consultation about Standard Assessment Procedure (SAP) which is a way of calculating the energy usage of homes in the United Kingdom and creates a methodology for doing so. The government are, through the Buildings Research Establishment, conducting a consultation about a draft set of procedures called SAP 2009, which might come into effect in 2010.

 Those who prepared SAP 2009 might know a lot about building energy saving homes but they do not have experience with solar thermal and their methodology for calculating solar thermal is wrong. In essence it looks to ensure that the solar panels generate as much heat as possible, rather than looking at the overall system. It is a bit like choosing a car because it has a very high top speed, which top speed is never capable of being used. Solar panels should be designed not to get too hot – otherwise they have a short life and fail. With solar it is not about how hot you get the panel but how you manage and usefully use the energy it generates. More than that, as an environmental product that goes on roofs solar thermal panels should be designed to require no maintenance and not break down but last as long as the roof lasts, before overhaul.  I have made the following submissions as Genersys’ submissions to the consultation process.

About Genersys

 Genersys plc is one of the United Kingdom’s largest suppliers of thermal solar panels. It exports solar thermal panels to many countries in the world and closely works with its European Associated companies and its subsidiary companies throughout the world. As a result it probably can draw on the greatest and most diverse knowledge about designing thermal solar panels and their performance.

 Effect of Draft SAP 2009 on Market

 The Standard Assessment Procedure 2009 will be an important document that will in effect dictate how homes and other buildings should be built by dictating a system of energy rating for dwellings.

Genersys through its customers has received feedback about the draft SAP 2009. It is clear that house builders anxious to be compliant with draft SAP2009 are already using its methodology to dictate procurement of energy aspects of dwellings, notwithstanding the fact that the document is still in its formulation stage and is widely being discussed, with the consultation and review of the consultation still underway. In these circumstances the procedures and methodology of draft SAP 2009 is already having a significant effect.

 Preliminary

 At Genersys we are concentrating our comments on the draft SAP2009 on its solar thermal aspects, because we feel that inappropriate principles are being employed in the draft. We hope our input will be perceived as useful and will enable SAP2009 in its final form to be, in relation to the solar thermal aspect, a better and more accurate document, founded upon principles that apply to solar thermal technology, instead of principles of energy saving and heat loss for homes.

 The starting point for our submissions is that solar thermal technology is a very low carbon form of heat generation[i]. Principles that govern the design of buildings to conserve energy are very different from the principles that should be used to design a solar thermal system. We feel that in providing for the solar thermal aspects of SAP2009, the principles that apply to good solar panel design have not been applied correctly, with factors that are not relevant to solar thermal technology being misunderstood or misapplied. Solar thermal heat generation is not a building envelope that needs to be insulated or a heat source that like a light switch can be turned on and off. It is about collecting solar radiation and then managing the energy that has been collected.

 First Submission: Dedicated Solar Storage  or Volume is not a solar thermal concept and is unclear in its meaning.

 No solar thermal company uses “dedicated” solar volume as part of its calculations or methodology. The phrase conflicts with definitions in various existing standards and compliance guides, such as BS 5918, ISO 9488, BS EN 12316-4.3, CIBSE Design & Installation Guide, Appendix H of SAP 2005, Domestic Heating Compliance Guide (Compliance with England and Wales Part L1A & L1B Approved documents) and CE 131.

However, our objection to the phrase is fundamentally founded upon a concept which it fails to recognise in relation to solar thermal technology. The implication is that solar thermal technology heats a cylinder (or part of one) uniformly so that there is a definite proportion of solar heated water available to measure. That is wrong.

 In fact an important principle in solar thermal technology is the storage of hot water that is stratified in heat terms. Domestic cylinders are (or should be) designed to have the solar coil at the bottom, a fossil fuel coil at the top, and a sensor pocket located mid way. If a pre heat system is used (not many of them are or should be used) the same principles will apply. In each case, whether or not preheat, the whole of the volume of the solar heat fed cylinder will be available for solar storage, with the hotter water at the top of the cylinder (from where the water should be drawn) and the cooler water at the bottom.

 Because solar heats by demand (although of course solar energy is not always available in sufficient quantity)

 From this it will be seen that the design of the cylinder is more important than the volume of stored “dedicated” stored water, whatever that means. The assumption should be that the whole of the cylinder is available for solar heated water storage provided that the soil coil is long enough and correctly positioned to encourage stratification of heat inside.

 Second Submission: Linear Heat Loss Co-Efficient should not form part of any algorithm

 The home building industry (reports one of our major distributors) is purchasing solar thermal panels on the basis that they have to “conform” to draft SAP2009, and have a linear heat loss coefficient of less than 4.

 We should explain that Genersys’s panels designed for domestic use (not for industrial use) have a linear heat loss coefficient of more than 4. This is not because we do not know how to design the (roughly) quarter of a million square metres of panels produced by our associated factories in Slovakia and Greece, or because we wish to save money on insulating the panel. There is no problem (as our competitors show) in reducing the linear heat loss to below 4, so why does Genersys require its panels for residential water heating to have a higher linear heat loss co-efficient that its competitors?

Different testing stations even though they test to the same principles have different results for the same panels. According to One test station Genersys 1000-10 panels has a heat loss coefficient of 4.954 while another logs this as 4.391.

 We fail to understand why it is assumed for the purpose of SAP2009 that a heat loss co-efficient of arbitrarily set at less than 4 provides better performance and greater carbon savings that a higher linear heat loss co-efficient because:-

(a)    Solar panels must be balanced; if they are designed to have a low heat loss coefficient then the panels and the systems will over heat regularly. Overheating will lead to high stagnation temperatures. This means high wear and tear on the absorber surface and failure on joints seals and valves caused by overheating.

(b)   You can design a solar panel with a very low heat loss co-efficient – it is not difficult – simply use very high insulation. However the wear and tear on the panel and the system as a result of heat loss would mean a short working life of the panel. This is important because all the panel testing only measures heat when the panel is new, not after years of working life.

(c)    Replacing failed panels and systems caused by overheating is pointless, in terms of carbon saving. Genersys panels are provided with a 20 year guarantee because they have a higher than average heat loss co efficient. If we were required to produce panels with a lower heat loss co efficient by the market’s reaction to SAP2009 then we would do so but (i) only for the UK market and (ii) only with reducing our panel guarantee from twenty years to ten years.

(d)   Keeping more heat within the panel will also lead to quicker degradation of polypropylene glycol used as a heat transfer anti-freeze fluid. This has carbon and cost implications for dwellings.

(e)   The difference in heat gained with a heat loss co –efficient of 5 or less than four is not significant[ii], because the solar system is available to collect energy in all daylight hours.

(f)     Far greater savings of carbon can be achieved by (i) properly and fully insulating the heat exchange pipe circuit to prevent loss of useful heat rather than calculate loss of heat that is not useful in the collector and (ii) specifying and calculated the heat loss from the cylinder; that is significant because it is useful captured heat losses that should be measured not theoretical uncollected heat.

 Third Submission; Efficiency at Zero Loss is also a concept that should not form part of the algorithm used under SAP

 The Genersys 1000-10 panel measures (in its latest test report) efficiency at zero loss at 0.776. The building industry is taking SAP2009 as requiring a “better” zero loss performance of 0.814 or higher.

In fact this is a complete misunderstanding of the efficiency of solar collectors which unfortunately SAP2009 is unwittingly encouraging. It should be noted that the output from the panel at zero loss efficiency is 1380 Watts.

 The efficiency of solar collector ŋ₀ is influenced by: Optical heat losses (absorption of selective coating and transparency of glass), these two values are NOT related to any temperatures

Thermal losses; which are dependent on delta T of absorber surface and air temperature around the collector.

 If  you heating a swimming pool, the mean temperature of glycol inside the collector (Tm) is almost the same as air temperature around the collector (Ta). (x=0). (fig.1) In this case the optical efficiency of collector with selective coating or without selective coating is similar or equal to 80% efficiency.

 Higher efficiency, around 90%, is achieved by unglazed collectors (plastic mats) because there are no optical losses. Lower efficiency, around 60%, is reached by evacuated tubes. Evacuated tubes have higher optical losses than flat plate panels which are caused by round sides of tubes and their lower absorber surface area.

                                     X =   Tm-Ta                            (m²KW^-1 )

                                                Gk

 Tm – mean collector temperature

Ta – air temperature

Gk – global radiation

 When you heat potable hot water, in most cases satisfying temperature Tm should be 35 -40 K higher then Ta which with average global radiation 800W.m^-2 which is equal to x=0.05. In case the efficiency of good quality flat plate panel (with selective coating) and evacuated tubes is the same, around 55%. Due to high thermal losses collectors without selective coating (cheaper flat plate panels) reach efficiency only 40% and lowest, 20%, is achieved by unglazed solar collectors.

 If you use solar collectors  to support space heating (low energy heating) delta T (Tm-Ta) is 40-55K, but the average solar radiation is only around 400-500 W.m^-2 In this case x = 1 and highest efficiency, 45%, is achieved by evacuated tubes or vacuum flat plate panel where the vacuum eliminates thermal heat losses. Good quality flat plate collector has at this point around 28-30% efficiency, which is around 2/3 of vacuum collector. Using low quality flat plate collectors (without selective coating) is inefficient.

 The real efficiency of solar collector is related to overall quality of solar collector, and the yearly working efficiency of collector is related to details such as quality of selective coating, type of glass, collector ventilation (breathing holes) and thermal insulation. Combination of all these matters defines difference between good and poor quality panel.

 The collector efficiency at zero losses is absolutely ideal state and it change within seconds. In terms of thermal insulation: if you review all above, unless you substitute insulation drastically, (e.g. mineral felt for vacuum as in the Genersys 1450), the overall efficiency is not influenced. The important thing is type and quality of coating on absorber.

 We have checked the specification of one of our competitor’s panel the Worcester Bosch. Worcester Bosch’s most popular panel has 55 mm thick thermal insulation, where the Genersys 1000-10 has only 40 mm. It is obvious why the Worcester Bosch’s thermal insulation is better.

 Most of the panels perform very well at their first year, but no one ever check how panels perform after years of usage. Genersys panels are constructed to last and provide sufficient output for years but are being penalised by using inapplicable SAP methodology.

 Comparisons

 We set out the key comparators between Genersys and Worcester Bosch to illustrate our point:

 

	Genersys	Worcester
Height (mm)	2040	2070
Width (mm)	1040	1145
Depth (mm)	85	90
Gross collector area (m²)	2.03	2.37
Aperture area (m²)	1.76	2.25
Max. operation pressure (bar)	20	6
Weight	36.5	41
Glass	4	3.2
Fluid content (Litres)	1.57	0.86
Zero-loss collector efficiency	0.814	0.77
Heat loss coefficient	4.391	3.681
Warranty (Years	20	2  or 10

 If the SAP 2009 calculations take account of Zero Loss Efficiency then the market will be pointed towards panels that have short working lives and are prone to overheating. Overheating is very significant in thermal solar systems for many reasons. For example regulations require three different safety devices in thermal solar systems to overcome dangers from overheating, even if automatic mixing valves are installed.

Conclusion

 Domestic water systems need to heat water to around 42ºC – 45ºC.   A good low carbon solar system does not seek to collect as much energy as it can, but to manage the energy that it collects most efficiently. Draft SAP 2009 is written to encourage the collection of as much energy as possible without regard to the management of the collected energy, its usefulness or the effects of overheating on solar systems.

 

 

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[i] Around 8 grams carbon dioxide per kWh using the same principles as used for establishing the carbon footprint of renewable energy generation by the Parliamentary Office of Science and Technology at http://www.parliament.uk/documents/upload/postpn.pdf  

 

[ii] Probably in normal hot water use within the arrange of 1% to 2%