PV-T Hybrid Solar. Separating Fact from Fiction | Anthony Morgan | Pulse | LinkedIn

”As many people will know I have been working on the development and delivery of PV-T as a technology and the associated systems for many years. In that time many mistakes have been made and lessons have been learned. In the early days, if I’m honest, I questioned whether the technology could live up to the early promise but what I discovered was not a flaw with the technology but an industry wide underlying misunderstanding of how PV-T and more surprisingly, how solar thermal technologies work. As time progressed and the number of working installations started to clock up, my belief in the technology and it’s ability to contribute to the new solar age has become certain. When it comes to diversifying from fossil fuels, I am a personal believer in a systems approach. No one technology will prove to be the Holy Grail, this is true on a macro level as well as on a micro level. When the combustion engine was invented, the engineers working on them knew that the efficiency of the engine was dependent on skill and workmanship that went into the production and engineering of the component parts. The same rules apply to developing a solution that offers a true zero-carbon approach and with this in mind and in the context of this article, a PV-T panel must be viewed as a component part within a wider system. Typical Hybrid PV-T and Heat Pump system, with ground charging Photovoltaic are typically 15-17% module efficiency, advances in technology will slowly push this up and we can expect the industry standard to increase by 3-4% in module efficiency over the next couple of years. Conversely, using the same testing criteria, solar thermal technologies are anything up to 90% efficient (for an unglazed collector, the sort used for a swimming pool). Both PV and Solar Thermal have an Achilles Heal, they both become less efficient with temperature. In order to achieve a near 90% efficiency from a swimming pool collector the water passed through it must be close to ambient. This statement is true for all types of solar thermal, Glazed and Evacuated Tube, however their efficiencies are closer to 83% and 77% respectively. The truth of the matter is that a Glazed flat plat collector will displace far more energy when used for low temperature heating applications than it ever could for heating hot water. In other words the industry have it wrong, we should be developing solar solutions designed to keep cool and to achieve this we have to think in terms of systems. Temperature vs efficiency of different solar thermal technologies (also referred to as Tm-Ta). The bottom line denotes stagnation temperature, or the temperature at which a collector radiates as much heat as it absorbs. Collector efficiency, is the amount of the suns energy converted into heat. Critics of PV-T have said to me on many occasions, that PV-T dose not make sense as the thermal gets hot and degrades the output of the PV. This statement only goes to highlight the misunderstanding within the market. Using low grade thermal energy makes so much sense, as the closer to ambient you get the lower the energy loss due to heat transfer, the less energy loss due to wind and weather and the more kWh of energy are harvested. At the end of the day we are all interested in displacing kWh of energy currently dependant on fossil fuels, the tank of hot water we need to take a shower is a product of a process using energy, whether it comes from fossil fuels or the sun. So back to efficiency, a well designed PV-T module in a well designed solution is able to convert up to about 70-80% of the suns energy into useful energy. You may say “how is that more efficient than a 90% swimming pool collector?” simply put it’s not but in terms of kg of carbon displaced, it beats it hands down. This is due to the electrical element of a PV-T panel. We are only converting the same energy as a solar thermal panel but the carbon value of electricity a PV-T module generates is greater than that for heat, by a factor of about 3. So our 17% efficient PV is displacing as much carbon as a 54% efficient solar thermal collector of the same surface area (provided of course the building has somewhere to put the heat). But unlike a PV module we also capture thermal energy converting roughly 60% of the suns energy into useful heat too. At this point it’s worth noting that a mono-crystalline PV module of 16.5% module efficiency, tested under standard test conditions, would at 60C be converting about 14% of the suns energy, this loss is due to temperature degradation, so again the value of keeping the panel cool over time is clear. Graph showing electrical yield degradation due to increase in cell temperature OK, so having established that PV-T is a valuable tool in the delivery of a low carbon, solar economy, how do we engineer a solution to maximise the benefits of the technology? Energy does not disappear it merely changes from one state to another. This is an important fact to remember. If you put thermal energy into a ‘hypothetical’ storage medium which is perfectly insulated, that energy will remain until conditions are such that it is able dissipate to its surroundings and a temperature equilibrium is returned. On that basis, a non-perfect inter-seasonal storage of thermal energy is perfectly plausible. A large enough volume, at a low enough delta ‘T’, with enough insulation between the store and the surrounding environment will provide a suitable environment to harvest, collect and store energy for when it’s most needed. We have played with many ways of achieving this, from the use of phase change materials, storing energy deep in the ground, or, our favoured approach more recently of using the soil in footings of a building. Like an inverted swimming pool once the energy is there, the majority can be maintained for use later. Results of an actual ground source heat pump system using solar energy to charge the ground. The uplift in background energy resulted in a 25% increase in annual performance of the heat pump. Results courtesy of David Nicholson-Cole – Nottingham University Understanding this, allows us to take the suns energy when it is abundant and store it for use when we need it. Also, by discharging the thermal energy over a winter period, the store is coldest a good three months after the sun is at its weakest, so we are able to start the summer charging season with our PV-T at its most efficient. In this set up we are able to maximise the energy harvesting potential of the PV-T. I hear you ask; “All this low grade thermal energy, what about having enough heat in my tank to take a reasonable shower?” This is not unreasonable and nor would I expect anything less in our comfortable modern world. The trick here is the correct integration of heat pumps with PV-T. As stated before, PV-T is a component within a system, heat pumps take an abundant source of low grade thermal energy into a gas cycle and compress it to provide a smaller volume of gas at a higher temperature. Unlike solar technologies, heat pumps drop in efficiency as the source temperature drops. More accurately put, as the differential between the source and the target increase, the heat pump efficiency decreases. So, through solar charging, we can efficiently increase the source temperature thereby significantly increasing the heat pump efficiency, taking all that low grade thermal energy harvested from the sun and then upgrading it to raise its temperature and ensuring you get that nice warm shower you are used to. Heat pump efficiency curve showing the relationship between the refrigeration gas temperature on the source side of the heat pump and COP (Coefficient of Performance). COP is the relationship between electrical energy being used to drive the heat pump against thermal energy out, i.e. a COP of 3 = 1 unit of electricity being used to produce 3 units of heat. In a well balanced PV-T heat pump solution there is an added advantage that the electrical energy being produced by the photovoltaic element of the panels is used to displace the electrical power required to run the heat pump, further reducing the amount of grid power required to run the system. A synergy of systems perfectly designed to exploit one another’s properties to maximise efficiency, much like the engine of a well tuned car. Our systems are exploiting the suns energy in much the same way that the engineers of the 20th Century exploited the locked up energy in fossil fuel. For some systems like swimming pools, the target temperature is such and the quantity of energy needed, that storage becomes irrelevant. There is no question for such applications the cost benefits make the application of combined system a no brainer. Why install large fossil fuel combined heat and power plants when you can have a system that provides the energy from the sun for free? Let’s face it, using fossil fuels in the combustion process is just so last century!” By: Anthony Morgan – Head of Minimise Generation Written by Anthony Morgan + Follow Anthony Morgan Co-Chair, Director PV-T Hybrid Solar. Separating Fact from Fiction


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