Friday, 19 October 2012

Hybrid PV-Thermal Solar Panels - A Good Idea?

The following article is a summary of a more technical paper, which can be downloaded here:

Solar Thermal Panel Operating Temperature During a Calendar Year

Working for a manufacturer of both solar thermal and solar photovoltaic (PV) panels, I am often asked why we didn't combine both technologies into a single panel.  Then we wouldn't have had to spend so much time and effort making our PV and themal panels match so they look great together on the roof.  And what's more, (they continue), a hybrid PV-Thermal (PV-T) panel is better because the solar thermal removes heat and cools the PV cells to a lower temperature where they work more efficiently.

I'm going to share with you some of the work we did that informed our decision to keep solar thermal panels and solar pv panels separate. 
You see, the notion that the solar thermal part of a PV-T panel keeps the PV part of the panel cooler ignores the whole point of a solar heating system - for it to be useful it must increase the temperature of something.
The most common "something" to be increased in temperature is a tank of water for use washing and bathing in a residential building.  For sure this starts the day at a cool temperature, but if the 'T' in your PV-T panel is to be of any use, then it will finish the day at a warmer temperature.
The image at the top of the page shows the operating temperature of a solar thermal panel heating domestic hot water in three minute time slices for a whole year.

As each day starts (top of the image), the panel is working at low temperatures (blue/green colours), but for days with good light levels, the hot water cylinder heats up and the panel has to work at higher and higher temperatures (red/orange colours) to keep on adding heat to the hot water cylinder.  The higher temperatures reached as we move from January (left of the image) to the longer days of the summer (middle of the image) are also evident.

Also of great relevance is that the thing you are heating often has a maximum allowable temperature.  For example, when heating a hot water store it is common to stop at 60-65C to avoid scalding risks.  Once this happens, the solar thermal system is not circulating the "coolant" to the panel and the insulation that makes it thermally efficient means the panel gets very, very hot - temperatures around 220C are common.  Aside from the effect of these super-high temperatures reducing the efficiency of the PV cells in a hybrid panel, the effect of such temperature cycling of the PV cells and solder connections seems unlikely to be beneficial to the panel life expectancy .
By contrast, here's the temperature for a south facing PV panel for the same year of weather data using the same temperature colour map:

Temperature of a PV panel over the same calendar year of weather data

Like the thermal panel the PV panel starts cooler in the morning (top of the image) - at ambient temperature, then raises to the afternoon as more and more light falls upon it, before falling again as we progress to the evening (bottom of the image).

It's clear just comparing the two images visually that a solar thermal panel runs hotter than a PV panel, but here is a plot comparing the temperature of the two panels over the year:

Plot showing when a thermal panel would be cooling (blue/purple)
or heating (orange/red) a PV-T hybrid panel

Blue and purple pixels show times when the solar thermal panel is operating at a temperature below the PV panel, and would therefore be cooling a PV-T hybrid panel.  Red and orange pixels indicate that the thermal panel is operating at a temperature above the PV panel and would therefore be actually heating a hybrid panel and diminishing its electrical output compared to a stand-alone PV panel.
White pixels are where there is little difference.  Black areas are where the solar thermal system is not circulating, either because the panel is cooler than the hot water cylinder or because the hot water cylinder is at its maximum temperature (65C in this case).

Note that the red pixels are where the 'T' is heating the 'PV' to a temperature more than 10C higher than it would be in a standard PV panel.  This corresponds to a reduction of electrical power output of more than 5%.

It is possible to imagine a solar heating application which would keep the temperature lower for more of the day.  For example PV-T panels heating water in a swimming pool would operate at 30-40 degrees all the time. 
For the majority of applications though, the intuitively appealing idea that the sum of the whole is greater than its constituent parts turns out to be a mirage.  A heat-haze if you like.
This article is a summary of a more technical paper, which can be downloaded here:


  1. Thank you for sharing the temperature data. If you look at the temperature performance of a PV panel, most show about a .4% drop in output per degree. To expand on your illustration, if you wish to heat a cylinder to 65c, then your panel will be at 75c to allow for proper heat transfer. If a PV panel was at 40c, by adding the PVT and running it at 75c will reduce its output by 35*.4%=14%. This will not be for all the day, but is still a significant reduction.

    There is another factor that you have not mentioned, sizing. A 4Kw system is about 28m2. A typical flat plate solar thermal system is about 4m2. Either you only install some PVT and the rest PV, or you have a system capable of heating a pool (and outside RHI due to SAP). If the client wants all the panels to look the same, there out PV & ST panels that match (from a number of suppliers).

    Other issues that I can see are: who installs them? PV or ST engineers, and is there not a conflict with MCS that states that PV panels are installed to allow for cooling.

    If I have misunderstood, can someone please explain where the maths is wrong?

    1. Geoff, if I have understood your q correctly you seem to be implying that adding PV would increase the panel temperature, but that is not the case, it would only cool

      also pool heating is allowed under the commercial RHI, as long as it is indoors.

      an ideal application would be pools because you would size PVT for maximum thermal benfit and the PV would be a "bonus". My understanding is that a PVT panel performs about 80% the value of a standard PV or thermal panel

      Ben Whittle
      Green Earth Energy

    2. Geoff,

      good point about the relative sizes of solar thermal installations and PV panel area.

      For a typical domestic installation heating hot water only, you need around 3-4 square metres of solar thermal. More area increases the energy yield only slightly, in summer it results only in the panels spending longer in stagnation. (The hot water cylinder can only hold so much heat, demonstrated by the images in the article).

      The average PV installation is 2.5kWp according to the Solar Trade Association. This would be some 15 sqm.

      So the designer seeking to use PV-T on a domestic property faces a stark choice

      - size it for the heat load (and get a sub-kWp PV system)
      - size it for the desired PV output (and spend longer in stagnation, massively overheating the PV in the panels), or
      - mix PV-T with standard PV modules, incurring greater complexity on the electricial installation side as the PV-T panels will not electrically match the PV moudules and should not be wired into the same string

    3. Make a setup where after warming a hot water preheat tank, before returning to the roof, it runs through PEX tubes in the ground under the house. I've used the PEX ground heat storage to reduce attic temperatures with a large radiator made from baseboard finned tubes soldered together and a 20" window fan. After several years, the highest return water temperature (Aug-Sept) has been 78 degF.

    4. "Anonymous" is doing interesting work. If the goal of improving PV performance by managing temperature is selected as the priority, then it's probably necessary to find valuable uses for the heat other than domestic hot water. Storage in a large mass for use during cooler months such as in the ground under a building sounds very promising to me. Being new to this sector I'm keen to read as much as possible. Appreciate what has been written here... Thanks!

  2. Nice post! Really clear explanation. I have been interested in this for a while and fielded a few enquiries about it, but I have remained slightly sceptical, and drew the same conclusions as you about swimming pools etc.

  3. this is interesting. thank you for this information on solar pv, phil

  4. Solar PV is relatively cheap to install and very easy to maintain. You may spend a few hundred dollars to get everything in place but besides that, you can sit back and watch your energy costs decrease.
    Solar Installers

  5. I am wondering about the feasibility of there being value in ducting cool exit air from a home evaporative cooling system with air at 15-20Deg C below ambient across a PV array? This would be a relatively cheap capital cost and zero operating cost for a 7-10% output increase

  6. I’m afraid although the steps followed make sense there are some flawed basic assumptions in this article.
    1. The assumption that the PV-T panels will reach 200 degrees C. There is no manufacturer who would design PV-T panels to stagnate at this temperature, for a start the PV would de-laminate and become useless and as such a panel designed in this way would never pass the engineering standards for PV and solar thermal , (IE. EN 61215, EN 12975-2, EN 61730). And so could never attain MCS accreditation.
    2. “You see, the notion that the solar thermal part of a PV-T panel keeps the PV part of the panel cooler ignores the whole point of a solar heating system - for it to be useful it must increase the temperature of something.” You can increase the temperature of something at the same time as cooling the PV panels, in their own modelling it shows the PV panels reach 70-80 degrees C. The PV-T panels are designed to be less thermally efficient than a standard solar thermal panel with lower stagnation temperature. If you try to use a PV-T panel in the same manner as a high temp solar thermal panel (feeding a cylinder with high temp system boiler also feeding) it will not work properly.
    The main problem with this article is the base assumptions are incorrect, I.E a PV-T panel is not the same as a high efficiency solar thermal panel and a PV panel.
    Smaller hot water systems must be designed to suit the buildings summertime peak requirement and usually use PV-T with a slightly higher stagnation temp (Glazed). A good Glazed PV-T panel will give you roughly 80% of the thermal output that the same M^2 of glazed flat plate solar thermal would give you as well as giving you roughly the same annual production of electricity per kW peak installed as PV. This means that in the same footprint you are generating more useful energy than either solar thermal or PV collectors. This in turn leads to higher displacement of CO2 per M^2 of roof space than either PV or solar thermal which is becoming key as the Code level rises.
    Larger systems would be usually more electrically focused, in these systems a PV-T panel would usually be selected with a lower stagnation temperature (Un-Glazed). These panels give bags of Kwh but at a low temp so they are perfect for low temp applications, ie swimming pools, pre-heat systems, as a source for a water to water heat pump etc. Depending on the application (and how cool you keep the panels) will determine the annual increased electrical production of the panels per kw peak installed above a standard PV Panel. In the case of swimming pools, assuming the panels are working on a simple differential controller the panels will be passively cooled whilst heating the pool, this should mean the panels never really get above 35-40 degrees C. As this is the case all of the points on the PV temp graph where the colour is yellow to red now become points where the PV-T is generating more electricity than a standard PV panel would be generating (in cases such as this we find PV-T will produce roughly 10-15% higher annual yield than the same KW peak installation of PV)

    Continued below

  7. Continued

    In the case where the PV-T panels are being used as the source for a Water to water heat pump there are even greater advantages to be had. Although the panels stagnate at a lower temp then standard solar thermal if the sun is out they are hotter than ambient. When the panels are used as the source for the heat pump the COP of the heat pump is increased (I.E greater thermal outputs vs electrical input). At the same time rather than passively cooling the panels you are actively cooling them, as this is the case you can maintain an even higher electrical efficiency of the panels. This of course is useful only if you have a use for the upgraded heat produced by the heat pump, i.e. heating a property. It should be noted, the free solar thermal energy from the panels will be at its maxima in the peak summer when there is no demand for the heating to be run. There are ways to deal with this, one of which is to use a multi-source heat pump system. In this type of solution the PV-T is tied in with a borehole system, the heat pump can choose the hottest source to run (solar in the day, borehole at night) and always prioritise free solar gain. The benefit of this system is rather than creating cold bubbles in the boreholes one can re-charge them using the excess low-grade thermal energy from the PV-T panels, ie. If cylinder is satisfied and panels are hotter than borehole divert to borehole. This not only recharges the ground but also cools the return to the panels therefore maximising the Electrical output. These systems are not theoretical or make believe, they are installed here in the UK, controllers have been specifically designed from scratch to optimise these types of hybrid systems, the installations are exceeding the predictive models and moving at an accelerating rate into the main stream.
    I am aware I have gone slightly off on a tangent here and so to return to the point. The Author has made some fundamental misassumptions in their theoretical design of PV-T panels, i.e. If one was to make a high- temp PV-T panel, it would not function in the desired manner. However one would not make a high temp PV-T panel for exactly this reason. The system design is Key in putting together an effective PV-T solution, this does not need to be as complex as people assume. With rising code levels and higher carbon displacement targets PV-T will become more and more prevalent in the market. As with any new technology there are people who will sell the technology without understanding it and the systems will not perform as well as they could, but there are also people who will be able to provide tailored effective solutions. The true benefit of PV-T is that you can generate more energy per m^2 of roof space than anything else on the market, meaning greater generation with less roof space, single panel type installed and peak capacity installed for less than an equivalent solar thermal and PV array.

    1. David

      Many thanks for taking the time to respond to this article. You seem to have expertise in working with combined PV-T panels.

      It would be really helpful if you could expand on some of your points, preferably directing us to evidence where possible.

      1. Limiting ourselves first to a simple domestic hot water system with PV-T panels. Your glazed PV-T panel is for heat-led applications. You have deliberately de-rated the panel to reduce the stagnation temperature, but this alone will not guarantee that the temperature of the hot water cylinder does not rise faster than a standard PV panel would heat up. Especially on days with high irradiation, the volume of water to be heated (which in turn depends upon the hot water use of the building) and the area of the solar panel installed will determine how fast the temperature of the store (and therefore the operating temperature of the panel) rises during the day. You claim that a PV-T system will produce 80% of the heat output of a same-size thermal system and 100% of the electrical output. How were these figures arrived at? How do you ensure this when the hot water use of a household is so difficult to predict and in any case changes over time? If the PV-T panel is large compared to the hot water demand (irrespective of it's slightly lower efficiency), surely the PV output could be *very much* reduced compared to a standard PV module?

      2. I agree that a low temperature applications such as heating swimming pools make a lot of sense for the technology, and stated this in the article. Do you have any evidence to support the claimed 15% improvement in electricity yield, or is this modelled?

      3. I agree that attaching the PV-T to a heat pump is likely to be of limited value due to the demand for space heating being at the wrong time of year for any cooling of the panel to have a significant effect on electricity yield.

      4. Re-charging a GSHP bore hole during the summer is an interesting concept that would be of value to standard solar thermal systems too. I believe this area deserves more research. Do you know of any evidence or results from such systems? How much of the heat placed during summer months is retained by the time you want to extract it in winter? How does this vary with the subsoil/groundwater conditions?

      Thanks again for contributing to the discussion! (BTW - it would be helpful for readers if you were to declare your affiliations).


  8. Hi Stuart,
    I apologise for not getting around to responding until now and for neglecting to mention my affiliations. My name is David Browne and I am the Technical Director at Newform Energy.
    Newform Energy specialise in the hybridisation of renewable technologies in order to create more efficient systems. We were the first to get MCS accreditation for both solar thermal and PV on a PV-T panel and now have around 200 installations across the UK. We have worked with most types of PV-T panel and above all we have found that system design and control is key to getting the most out of a panel. I cannot pass out customer details on a public forum, if you or one of your engineers from Viridian solar would like to contact us then we can arrange for you to pay a site visit to some of our customers to verify the information below.
    In response to point 1, yes you are right in the fact that the rise in temperature of the panel is dependent on the start temp of the tank, size of the tank, hot water usage and area of panels installed. However you are wrong in the assumption that the PV-T cell temperature would rise faster than the PV. As long as the tank is at a reasonably low temperature to begin with the surface temperature of a PV panel will rise a lot faster than the PV-T because of the thermal mass, ie. The PV just has to absorb enough energy to heat the panel up to its stagnation point but the PV-T has to absorb enough to heat the panel, fluid in the system and whole tank.
    PV temp:
    PV-T temp:
    The thermal side is much the same as any solar thermal system, you design the system to minimise the summer time stagnation by sizing the surface area installed by the requirements and tank size. The only difference is, as the panel is de-rated when it does stagnate, it does so at a much lower temperature than any standard high temp solar thermal panel. The figures stated previously for the glazed panels come from customers outputs. We were lucky enough to have a customer who wanted to have 9 glazed PV-T panels installed on the same roof as a PV array, all the panels are at the same tilt and direction and all with no shading. He has forwarded us his outputs and there is a couple of percent difference in the electrical kWh produced per kW peak installed.
    As I am sure you know correctly predicting the solar input from any solar thermal system is vague at best, with customer usage playing such an important role. I can forward the Engineering test data for some PV-T panels to you if you like which have the zero loss collector efficiency, a1 and a2 co-efficient calculated and then you can put this into whichever software you use for solar thermal estimation, obviously the contribution varies from panel to panel but the ones I referred to tends to be around 80% of an average glazed flat plate.

  9. With respect to your last point in 1, yes of course, if you oversized the glazed PV-T panels to the houses hot water demand then the electrical outputs would be affected, however this would be a system design error and would have no reflection on the technology. For instance if I massively oversized a solar thermal installation then the amount of useful solar thermal per panel would be reduced the extra thermal stress would mean the panels lifetime would be shortened. This would not mean the panels were an ineffective product, it would simply mean that I was inept at designing solar thermal installations.
    In response to point 2, We have had energy companies send out engineers to test customers systems due to the high kWh generation per peak installed capacity. We have output data which we will be happy to share with you but it will only show improvement against predicted PV outputs as the end customers in general do not have PV arrays beside the PV-T arrays. We currently have some PV-T panels being tested by a utilities company, who are testing them beside the actual PV modules that are used within the panels. The tests are being conducted with all panels in the same plane, using identical ancillary components. This should be a fair test, although as you know every user will have a different thermal load and so the profile for the thermal side of the testing will directly affect the electrical outputs of the system. As soon as we have access to the data from these trials we will be happy to share them with you. So in short, to answer to your question the ~ 15% uplift comes from actual outputs above the Predicted PV outputs.
    In response to 3, If you size the system right then using PV-T panels as the source for a W2W heatpump has multiple benefits:
    • Hotter source for the W2W heatpump, as a result it will run at a greater efficiency
    • Cooling panels so maintain a higher electrical efficiency on the panels.
    • Running heatpump midday into thermal store makes use of electricity otherwise fed to grid.
    • With boreholes, some inter-seasonal energy storage.
    • With boreholes once cylinder is satisfied can divert and charge boreholes keeping panels cool and maximising electrical output.
    In these types of systems there would be no point in installing them if you were just after the electrical output, they come into their own when used in a more holistic approach. When you have the requirement for heating hot water and electrical generation, it makes no sense to use one technology for one, another for the next and a third for the final. A correctly combined system will give efficiency benefits by exploiting the symbiotic relationships of the technologies being applied. This will further reduce household bills and CO2 generation compared to using separate stand alone technologies.
    In response to 4, I am glad that you think that the ground charging is interesting, it has been an area we have been trying to get more people to look at for a while. We currently only have around 6 residential installations with boreholes included and one large commercial. As you know under set conditions the lower you can keep the average fluid temp in a flat plate collector then the more kW you will get from it. Charging boreholes then becomes a perfect application for PV-T and solar thermal. If this is a subject that interests you then we can arrange for you to visit a couple of installations if you like.
    Below is the modelled effect of ground charging with the CIBSE image, then below that is the recorded effect of ground charging which is greater than the modelled.
    Another project to look up would be the Canadian solar charging project Drakes Landing. If you do wish to view any installations then please contact the office at Newform Energy.
    Thank you Stuart and I once again apologise for the delay in getting back to you.

    1. >As long as the tank is at a reasonably low temperature to begin with the surface temperature of a PV panel will rise a lot faster than the PV-T because of the thermal mass, ie. The PV just has to absorb enough energy to heat the panel up to its stagnation point but the PV-T has to absorb enough to heat the panel, fluid in the system and whole tank.

      This argument sounds like science, but is just utterly misleading.

      A heat capacity argument only works if the net heat flow into the two systems compared is similar. A PV-T module has features deliberately aimed at trapping heat, a PV module does not.

      Yes, the PV module is always at a stagnation temperature for a given level of irradiation, but because the thermal insulation of the PV module is negligible, heat losses balance irradiation at a comparatively low delta T. This is especially true early in the day when light levels are lower.

      What the analysis in the article shows is that for water heating, by the middle of the day when the stagnation temperature of the PV module has risen, so has the temperature of a hot water in the cylinder heated by solar, and more often than not to a level that would produce hotter temperatures in a “cooled” PV-T than in a stagnating PV module.

      Yes, by all means de-rating the thermal aspect of the PV-T compared to a standard solar thermal panel will reduce the rate of temperature rise of the heated volume unless, of course, the installer succumbs to the temptation to install a larger area of PV-T than the corresponding thermal panels. A temptation which must be very great because a four square metre thermal installation would be adequately sized for an average family home, but provide a rather modest 800Wp if the same area of PV-T was installed.

      >We were lucky enough to have a customer who wanted to have 9 glazed PV-T panels installed on the same roof as a PV array, all the panels are at the same tilt and direction and all with no shading. He has forwarded us his outputs and there is a couple of percent difference in the electrical kWh produced per kW peak installed.

      This positive example you provide illustrates that the glazed PV-T panels can produce an electrical output close to the performance of standard PV modules. Can you tell us what heat load there is on this property that means that it can keep 18 square metres of PV-T adequately cooled? How well does this example represent an average household?

    2. To explain the point about heat capacity further:

      Imagine putting two pieces of aluminium out in the sun. One is a square of ordinary silver kitchen foil, the other is a plate of aluminium the same size but 2mm thick and painted black. Which will be at the hotter temperature?

      The black plate of aluminium has a heat capacity (thermal mass) 20 times higher than the foil - there's more of it to heat up - but that doesn't mean it has a lower temperature.

      The silver foil may reach its maximum temperature sooner, but the black plate is heading towards a higher maximum, and will be hotter than the silver foil.

      You cannot conclude only from the heat capacity of the two dissimilar pieces of aluminium which will be at the higher temperature. The same applies to a standard PV module and a PV-T system.

    3. Hi Stuart,

      I am sorry for not getting back to you I just haven’t had the chance until now.
      I am afraid that your theoretical assessment is incorrect as it is not similar. The variation in temperature will take longer with the larger piece of aluminium and the instantaneous electrical efficiency is affected by the surface temperature of the PV. The larger volume of aluminium plate will not only take longer to heat because of its mass but also because you are taking the heat away from it which you have not taken into account.
      >This positive example you provide illustrates that the glazed PV-T panels can produce an electrical output close to the performance of standard PV modules. Can you tell us what heat load there is on this property that means that it can keep 18 square metres of PV-T adequately cooled? How well does this example represent an average household?
      The panels are smaller than standard PV, this would be around 12 square meters. As I have said before the glazed panels give you roughly the same kWh per kW installed as PV, the benefit comes from the fact that the solar thermal is in the same footprint, and these panels are not cooled. It is the unglazed panels which give you more electrical kWh per kW installed than PV.

      It seems a waste of both your time and my own to continue to discuss the theoretical performance of existing products when we can simply test them. As this is the case I would like to suggest an independently assessed comparison.

      I will donate some PV-T panels to be tested against Viridian’s aesthetic solution to differing panels on roof. To do a like for like I suggest one of your PV15 panels and one of your V15 panels against two PV-T panels. It would seem sensible to asses which combined solution gives you the greatest total generation per m^2 of roof space and which gives you the highest annual electrical yield per kW Peak installed capacity. If you like I will also supply the PV panel used within the PV-T panels in order to do a direct comparison of outputs electrically.

      This could be carried out by an independent body that we both agree on and then the results of the “comparison of the Viridian solution versus a hybrid PV-T solution” published here on your blog and on Newform Energy’s website.

      Would this be acceptable to you? I think it would make a lot of sense and would put to rest our respective opinions and replace them with independently determined facts.

      I hope you have a good Easter weekend

    4. You're on, let's do it!

      The test you propose is not a realistic assessment of the technology, and misses out a major element we have been debating - namely my contention that a PV-T system for a domestic building would not result in the T part cooling the PV part. Such a small working area is rather unrealistic for what people would want to install in the real world. The average PV installation is 3.5kWp according to the EST.

      Let's instead take the example you have given. 12 sqm of PV-T on a domestic property. It is my contention that this area of PV-T would spend most of the summer in stagnation (having met the heat demands of the house by mid-morning). The high stagnation temperature will severely compromise the output of the PV element compared to standard PV panels. I look forward to being proved right!

      Here's my proposal for a meaningful test.

      We each have 12m2 of "roof" area available and a hot water demand underneath it equal to the average domestic property.

      For the water demand I suggest we would follow the same tapping cycle as we used in the independent test of the Viridian Solar Clearline thermal panels by the BRE - EU M324 EN Tapping Cycle 2. This is equivalent to 100 litres per day at 60C.

      See the following link for details of the testing we already undertook on Clearline solar thermal panels:

      Viridian Solar - Clearline Solar Thermal Test Report - Average House Simulation

      David, please contact me at in the first instance, and we'll make arrangements for the Big Match...

    5. Well I have to say I am fascinated by how this might pan out... my suspicion is that Stuart is going to be correct for the described tapping cycle, but theres only one way to find out!

      Please keep us posted... maybe a youtube video or similar?

      I'm sure we could all learn something from this test

    6. If you are going to the trouble of creating this test rig I would also like to propose another test for a subject which is rarely discussed in the UK - the maximum store temperature set on the solar controller

      It is common on the continent to run unvented stores at high temperatures and mix down to 60 using a TMV, whereas the UK is dominated by vented cylinders and solar controllers set to heat the store to 60 and then stop because a TMV is not practical to fit due to restricted head in the vented system.

      How about a test to see how much extra can be gained by following the continental method (once the main test has been completed?) using this tapping cycle as well?

    7. Hi Stuart,
      I just wanted to post a quick note on here to keep anyone following this thread informed. Stuart and I are now in agreement that there will be a comparative test undertaken. We are currently talking to some independent test bodies and discussing the test size and set up. Once we have some more definitive information we will make sure to keep anyone notified.

    8. great stuff, glad you are both up for this test I'm really looking forward to seeing how it all pans out..

    9. I am also looking forward to seeing the outcome of the test. I'm think of building at some time in the next few years and am thinking of installing a PVT system.

      On a connected issue, in the UK due to the prevalent cloudy conditions would amorphous silicon produce more energy due to its ability to absorb more blue (diffuse) light? Also is it true that the output of amorphous silicon cells doesn't drop off as much due to high temperatures that panels reach under bright light conditions? Would these factors not make amorphous silicon a suitable choice for a PVT system operating in in a location where it is normally cloudy.

      I'm a newbie to renewable engergy so apologise if my questions are stupid.

    10. Any update on the comparative test that was being set up?

    11. I've been trying to get the new National Solar Centre interested in the study. They've been busy getting set up and I hope they will have time to consider my suggestion now.

    12. Excellent, please keep us informed on any progress.

    13. I'll be very very interested to see the results.
      I have both solar thermal and PV installed. The thermal never going into stagnation because I heat dump into the house foundations.
      As a theory, I totally support PV-T, if it is done correctly, and have been looking at the hybrid panels as well as contemplating making my own version for another project I might be involved in.
      In terms of PV, using thermal for cooling and increasing efficiency, it should be very possible to do a beneficial cooling job, even at higher temperatures, because a PV panel mounted on a roof naturally gets heat reflected back at it from the roof, which in summer time in the UK can get quite hot (i.e. getting over 40 degrees C even before you mount a panel on it, which naturally restricts airflow and creates some heat in operation). So for a low temperature system thermal it is pretty much a no brainer that there will be benefit to the PV. However, for a higher temperature system, if you run evacuated tubes and have the manifold above the PV, you should theoretically be able to run the PV proportionately cooler and yet have the thermal output run warmer because the evacuated tubes would still be pulling the heat away from the PV so improving both in relation to ambient temperature.
      Of course, the main determining factor as to whether any of this is worthwhile is actually the cost (financial and environmental) of making such a system compared to the return achieved!
      One also wonders, do you gain further benefit from mounting a double sided PV in front of the thermal?

  10. And that means, more area == fast heating. Reliable work ...

  11. This comment has been removed by the author.

  12. Maybe I missed something here, but the water input from a hot water storage tank would be hotter than cold water that would normally enter the hot water heater. Therefore, water for example at 100 degrees would be input into the tank rather than water at ambient temperature. So, the water would need to be heated from 100 degrees to say 140 degrees rather than from about 60 degrees to 140 degrees. The entire argument makes no sense to me.

  13. I think these make sense if you are going to put in a heat pump anyway and live somewhere where the noise from a air source heat pump is an issue.

  14. Just wondering if there has been a result to this face off?

  15. Also interested to hear what the results from this testing were!

  16. Thank you for sharing the temperature data. I want to have some information about the size and the water tubes in PV-Thermal solar panels.

  17. I am getting ready to build and this discussion was great, until I got to the end and the results weren't in? Are they published somewhere else? Is the test ongoing? Where is it at?

    1. Hi John (and others keen for an update)

      We were hoping that the new National Solar Centre would host side by side testing. Unfortunately, although this initiative has shown some interest in the idea, it seems to be taking quite some time to find its feet and get up and running.

      In the meantime, Viridian Solar Clearline solar thermal panels have already been independently tested by BRE:

      And standard PV systems have well-described characteristics.

      There's no shortage of evidence on the performance of separate PV and thermal panels.

      Noone disputes that a PV-T panel would produce more electricity if the water that it's heating is always cold. The question we have not yet got a proper answer to is just what temperature would the water be in a domestic setting, where the only heat demand during the main solar production period is for domestic hot water.


  18. Hi Stuart, thanks for that and the link. As we are in a semi rural area of Australia, we need 120,000L water tank and plan to install a pool as we'll, so no shortage of heat sinks once the HW is hot.

  19. I read this thread and would like to give my humble opinion on this one. I'm a graduate student from the K.U.Leuven and I have been doing research on PV-T panels the last year for my master thesis.
    I don't know how much familiar you are with the concept of exergy? But if one talks about PV-T panels, one should talk about exergetic performance of the PV-T panels or about an overall electricity/gas consumption in the whole heating system in comparison with PV,PT or the 2 laying next to each other.
    The results of my research will soon be published(or I can already mail you my paper), but my conclusion for a water-cooled PV-T panel with glass cover and an area of 5m^2 which only delivers heat for domestic hot water production, is the following:
    -From an exergetic point of view, the PV-T panel always produces more exergy than a PV-panel with the same area. During summer the difference is the biggest, because the share in thermal exergy significantly increases with increasing irradiance.
    -The volume of the storage tank has neglible effect on both electric and thermal exergy production if you do a simulation over severall days and the storage tank is well insulated.
    -The optimal inlet temperature of the panel should increase together with the irradiance to increase overall exergy production(thermal+electric).
    -In the operating point for maximal exergy production the PV-cells from the PV-T panel have a higher PV-cell temperature than those of the PV-panel as a consequence of this optimal inlet temperature.
    -The concept of PV-T, at least water-cooled with a glass cover, will mainly be interesting in sunny climates in comparison with PV.

    These were the key conclusion from the research. :-)

    1. Dear Jean

      Thanks for sharing the results of your masters thesis. Yes, please - I'd love to see your paper (

      A couple of observations and questions:

      1/ Your conclusion matches my own in the original post - claims that PV/T panels result in a cooling of the PV module are not true when attached to a domestic hot water system, as the panel must operate at a temperature to heat the water to a useful temperature.

      2/ Your conclusion also supports David's contention that a PV/T module will produce more energy in total than a PV module alone.

      3/ Question - does your exergy approach take into account primary energy consumption avoided, or just energy produced? What I mean by this is do you value a unit of electricity as equal to a unit of heat? If you are considering primary energy use avoided, what generation mix did you use for electricity?

      4/ Question - I was a little concerned to read 'optimal inlet temperature'. Was your modelling based on an energy balance on a hot water store with real weather data and an assumed pattern of hot water draw off and back up heater input? Or was the model simplified to consider the performance of the PV/T panel under a variety of fixed inlet temperatures? My point is that in real life you don't have control over the inlet temperature to set it to an optimal level - the inlet temperature in real life varies based on the interaction of the above parameters. If you have a sunny day and a low water use that day, then the operating temperature the next day would be higher than if there was a lot of hot water use the previous day.

      5/ What area of PV/T panel were you modelling? There is a temptation to install a PV/T array sized to give a good PV output (3-4kWp), but this is then highly over-sized for the domestic hot water use, resulting in even higher operating temperatures as the system over-produces heat for the load.

      Looking forward to hearing more about your research.



    2. Hi Stuart,

      1/Depends on the heat load and irradiance. Given a higher cell temperature than for PV, doesn't mean the hybrid concept is useless.

      2/ Based on energy performance the PV-T will almost always win because you value the thermal energy as electric energy. For very low irradiance, there will be barely a difference.

      3/ I integrated a heat pump in the heating system so I could compare PV with PV-T in electricity savings per year. To assess the exergy of heat, you have to multiply the heat with a Carnot factor.

      4/That is true. I used wheater data from 2010 and a realistic hot water consumption. But in a first step I defined an optimal case: I did an optimal control for the panel as if it were standing alone, so considering inlet temperature freely controllable. In a second step I coupled the panel to a storage tank and compared its performance with the optimal case. Conclusion was the performance of the coupled panel was only slightly worse(3%) than that of the optimal case.

      5/I only modelled a panel of 5m2 because of the problem you mentioned.

  20. Jean, could you kindly post a link to your papers?
    thanks, mitch (email google-able "gusat")

  21. Could we get an update on where this is at as I am very interested in this discussion.

  22. I'm sorry to report that New Form Energy went into administration in December 2014.

  23. Hey Everyone,

    wonderful ocncepts here and interesting doscussion, Also, Jean, could you please mail me your papers? I am very keen on what you have found and the details.

    Id be highly grateful for it.

    Thank you.

    1. Has anyone done any trials with extracting heat from solar PV using a heat pump. Conceptually, it should be possible to regulate the temperature to achieve maximum PV generation, using cold fluid, while increasing the temperature of the fluid to do useful work using a heat pump.

  24. Has anyone tried/ heard of using PVT in combination with interseasonal heat storage (e.g. boreholes)?

    1. Funny you should ask. Our research group at De Montfort University is working with Caplin Homes who have applied PVT in exactly this way to a home in Leicestershire. More here:
      In our research, we plan to explore how the heat moves around the system during the day and year, as well as the optimum design of borehole system.

    2. Rick I also spoke to the director of Caplan homes about the New Form PVT panels and explained what I was doing with PVT in the USA, but never followed up.

  25. See and google for many others... albeit using 'normal' thermal panels, not PVTs - due to efficiency and cost.

  26. Great post. Thank you for sharing! I am loving the idea of the monitoring graphs!

  27. I have just picked up this long thread since it started and what I am about to share with all of you, including New Form Energy who along with a business freind of mine in Cornwall lost money trying to install one of the PVT systems, a director wanted installing on one of his holliday lets in Cornwall
    Unlike New Form Energy directors, I started in solar water heating in 1991, after developing a hot water tank, now sold as a thermal store by a majortity of UK companies since Nu Heat copied the concept of my first design, with Gledhill haveing copied the latest designs we sold for years under the name Powertech Solar until 2009
    I digress
    The first company to develop the correct PVT solar panel was Solar Zentrum in Munich in 2006.
    The then manager left the company in 2013 and like me, looked at all the problems with all the current PVT panels produced and started to focus on how to design a solar cooling/thermal panel separate to the PV panels
    My own design was in my head for 5 years, but haveing lost my Powertech company in 2009, my urgency was to start a new business free of staff and costs, by returning back to China where in 2003 I co designed and developed the Apricus brand of heat pipe tube collector.
    This superier heat pipe collector has been part of my solar central heating system in my Dorset house for the past 12 yrs with no service costs
    Last year with my exports growing again from China, my Chinese investor partner, along with an American investor put in the money to develop my thermaltricity self assembly brand.
    The failure of all PVT systems is those selling them, has no idea about thermal stores, working with none pressure solar systems where the thermal cooling part of the PVT does not increase in temperature above 30c
    Please do not tell me that 30c of heated water from5c has no value ? as it does, its 68% of your hot water target of 48c, that your body can except when you shower.
    My PVT thermal panel separate to the PV panel of any brand will be marketed in the USA next year from $80.00 per panel with fittings to distributors who invest in people training and except the PVT package we have developed over 5 years mainly in China.
    Where the country has no freezing weather conditions, we do not include any insulation as the panel needs to keep cool in summer months.
    Where there are freezing conditions and snow falls, we supply a 6mm Nano Aerogel insulation material with a 6mm water proof board to hold the aerogel in place.
    We also offer a snow melting device, as the potential in selling PVT panels into Ski Resorts across the world is massive, but as yet not tapped into.
    If you wish to know more, contact me at

  28. I read this article a few days ago and it didn't sit well with me for some reason. I'm no solar expert, so it took me a good while to pinpoint what I didn't like, but I think I have it.

    You have taken two technologies that have been optimised to work separately and you have attempted to mash them into one without any effort what so ever. This is akin to the argument that a hybrid car is bad technology because using a petrol engine to charge a battery to provide electrical assistance is just introducing more energy loss and weight into the system without providing any benefit. It would be terrible engineering to put a petrol engine from a Ford Focus and an electric engine from a Nissan Leaf into the same car because they have not been designed to work together. A Toyota Prius however can get excellent millage by charging its battery using energy from braking and then using that energy to drive an electric motor at low speed. It simply requires re-imagining how the two work together.

    In the same vein; PV panels sometimes get too hot to work efficiently. Thermal panels are designed to take heat energy away from a solar panel. Maybe there is some way to design a hybrid panel that utilises the heat energy from the PV panel in a productive way? Your argument certainly didn't provide any evidence to the contrary.

    I'm not saying you are wrong, I simply don't know. I am saying that your argument is disappointing and of no benefit to anybody. I haven't read any of your other articles and I do not intend to after this one.

  29. If you think there's a way for it to save energy, keep working at it.

  30. Just came across this blog, excellent info but it seems there is no real data to back up claims on either side - although in researching PVT there seem to be some new players coming out with what looks exciting kit, the likes of naked energy ( and who seem to have a number of case studies, albeit missing detail on data, as the above post points out it is not a case of dumping two technologies together without thought, work in this area has merit it seems and discussion should continue.

  31. We have a working system installed on a test house at De Montfort University in Leicester. The system comprises seven PVT panels plus one PV for comparison (using micro inverters), a low temperature thermal store for the heat collected by the PVT and a heat pump to raise the temperature of the heat to something useful. You can see the performance of the PV side of the system here:
    You can see the thermal performance here:

    If anybody wants to know more about this system or the results of far, email me on

  32. My partner and I are planning to build a passive solar house, and hope to include an indoor pool (attached separate building). I was thinking that PVT panels could work well for us, with the pool acting as a large thermal mass to absorb the heat, and the fact that I would not need or want high heat for the pool so the flow rate of the coolant could be kept high, allowing the PV panels to be kept cooler, e.g. under 50 Celsius, with the goal to keep the pool at around 30 Celsius.

    1. So, Steve, did you do this & did it work as well as you hoped?

  33. This comment has been removed by the author.

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