Solar Panels – Using Irish Energy.
The process that Solar Panels use to produce energy is something we are all familiar with already in Ireland. Solar Panels, evacuated tube solar panels or flat plate solar panels, operate on the same basic principal as a car parked on a sunny day. A car left under the sun will warm up throughout the day as the sun enters the windows of the car, heating the inside. The heated air inside cannot escape – not until you come along and open the door. This solar heat energy is effectively trapped within the car, raising the temperature inside far above that outside. Even in Ireland, the temperature inside can be many degrees hotter than that outside. This is the same way that solar panels work but in an optimized way. This means solar panels can produce carbon-free, renewable energy from an energy source that is ultimately free and ultimately Irish!
In the analogy above the car is functioning as an inefficient solar panel. Solar panels are designed specifically to exploit this effect. Solar radiation strikes the solar panel, entering through a glass shield before meeting a dark-coloured solar absorber. As anyone who’s ever worn black on a sunny day knows, dark colours excel at absorbing heat energy. As the solar panel is left sitting in the sun throughout the day, its temperature will steadily rise until it far exceeds the ambient air temperature. The absorber efficiency (n0) is a measure of the quantity of solar energy the solar panel is capable of absorbing, for a given amount of solar radiation.
The aperture area of the solar panels is the area of the solar panel absorber that is exposed to the sun and capable of collecting sunlight. The total amount of energy collected by the solar panels is therefore dependent both on the efficiency of the absorber, and the aperture area of the solar panels. The higher the n0 and Aperture Area values, the better.
Facing the Sun.
Proper facing is critical to the successful installation and operation of solar panels in Ireland. The sun will only ever be directly overhead when you are on the equator. In Ireland, because we are so far North, the sun will always be slightly to the South. Instead of passing overhead, it swings around to the South of us, moving from East to West. A solar panel will give its optimal performance when the sun is directly perpendicular to the absorber of the solar panel.
Therefore, to extract the most amount of energy out of your solar array, your solar panels must be angled towards the South as much as possible, to ensure that they are facing directly at the midday sun. We can mount your solar panels in many different places – on the ground, on flat roofs or off the gable end of a house. If a South facing aspect isn’t available, we can mount your solar panels in an East-West configuration, with one to catch the morning sun, and one to catch the evening. With careful design (which we are good at..) it is also possible to mount panels on either an east or west facing roof – anything is possible really except facing North!
As the sun will never be directly overhead, it will always be at an angle to the installed solar panel. Therefore, your solar panels must be properly pitched. By happy coincidence, the vast majority of household roofs in Ireland fall right within the ideal pitch-range for solar panels in Ireland – within 30 to 45 degrees for year-round use. A flatter pitch may give better performance from your solar panels in the summer when the sun rises higher in the sky. A steeper pitch will give better performance from your solar panels in winter when the sun is lower in the sky.
Optimal orientation of your solar panel can give an increase in yield of over 20% annually, compared to a flat or vertically mounted solar panel.
Keeping the Heat
A basic principal of thermodynamics is that heat energy always travels from hot to cold. The rate of energy transfer depends both on the quality of the insulation between hot and cold, and the temperature differential between them. Therefore, to prevent the ambient air from cooling the solar panel down again, the absorber must be insulated and sealed to prevent air exchange. Vacuum tubes further use a vacuum to prevent convective losses between absorber and the atmosphere. An important determiner of solar panel efficiency therefore is its capability to isolate itself from the outside air temperature, as the temperature within the solar panel will often be much higher. This gives the A0 value. The lower the A0 value, the more efficient the solar panel is at retaining the absorbed heat.
All this heat is collected up in the solar panel, but it can do no good up there.
All solar panels include some method to circulate a heat-transmitting fluid through the solar panel’s absorber. This circulation can either be natural – driven by a thermosiphon effect and the fact that heated fluids tend to rise and cooler fluids sink – or through the use of a circulating pump. The vast majority of solar panels installed by Hotfoot will use an electrically powered circulating pump to pump fluid up to your solar panel to be heated by the sun’s radiation.
Hot fluid then flows down through the flow side where it enters a coil at the bottom of your hot water tank. As the water in the tank is cooler than the solar fluid, heat is transferred from the Solar Panel to the water in the tank.
The Solar Cylinder itself acts as a store of heat, the same way a battery stores electrical energy. It needs time to charge up and get hot. It normally builds a store of heat through the early morning and afternoon, which is then released through the evening, overnight and into the next morning as hot water is drawn off. The Solar Cylinder must therefore store at least a full day’s worth of hot water.
As heat rises, there will always remain a store of hot water in the top of the cylinder, while cold water will be introduced into the bottom of the cylinder where it will be heated by the Solar Coil.
Heat will always try to move from hot to cold. Solar panels require a large amount of heat storage in order to be optimally effective. Hot water in your Solar Cylinder will be at a higher temperature than the surrounding air. The rate of heat loss will depend upon how well the material separating both air and hot water conducts heat. A bare copper cylinder wall is an extremely good conductor of heat.
It is useless to collect heat from Solar Panels only for it to be lost to the ambient air inside your home.
Therefore the entire solar circuit must be insulated. Both copper and stainless-steel solar cylinders are fully insulated to a dept of at least 50 mm, minimising heat loss as much as is practicable. All pipework is further insulated, reducing heat losses from the solar panel circuit. Even the solar pump station is contained within insulating foam.
Unlike conventional sources of heat, it is impossible to turn the heat source (the sun) off and on again to meet demand. It must be harnessed as and when it is available.
The most basic form of control is to mount the Solar Panel below the level of the hot water cylinder. That way, when the Solar Panel is heated, natural convection and circulation will cause the heated fluid to circulate through the hot water cylinder, then back down to the solar panels to be reheated. At night, with the tank heated there should be no circulation as the hottest point is at the top of the system. The drawback of this system is that it is otherwise uncontrolled. It will continue to produce heat so long as the temperature of the solar panels is higher than the tank temperature. This can lead to extremely high system temperatures on hot days when there is no hot water use.
The majority of Solar Panel systems will include a solar controller. This is an electronic control device which monitors the temperature difference between the bottom of the tank and the solar panel. When it detects heat available in the collector, it activates a circulation pump which pumps the solar fluid around the circuit, drawing heat down from the Solar Panels to your storage tank. The electronic controller also includes a special limit that will prevent circulation when the solar cylinder has reached a preset limit. Most solar controllers will also include a holiday mode, allowing the system to cool off overnight if there’s been no hot water use, minimising stagnation.
Stagnation occurs then the solar cylinder has reached its operating temperature, but the sun is continuing to shine on the panel. The sun can no more be turned off as it can be turned on demand. Therefore, we must include a method for dealing with this extra heat. The first option is to do nothing. The solar fluid will ultimately boil in the manifold, pressurising the system while preventing further energy transfer from the collector to the fluid. Solar Panels are designed to withstand stagnation temperatures and pressures without ill-effect. The system includes a gas-filled expansion vessel to absorb the changes in pressure. This will however, quickly reduce the effectiveness of the anti-freeze, requiring more frequent replacement.
Some solar panels systems will include what is known as a solar Heat Dump. This is a dedicated radiator controlled by a motorised diverter valve that will allow the system to continue circulating fluid once the storage tank has reached its limit temperature, preventing stagnation entirely and prolonging the life of your antifreeze.
What can actually be achieved with Solar Panels?
Our aim when designing a Solar Panels system is to provide 60% of your annual domestic hot water needs. This tends to exceed industry best practice which comes in around 50%. For six months from April to October, Solar Panels will ensure that you will only rarely have to light your boiler, or use your immersion heater during the summer – and even then your Solar Panels will do the majority of the work. Instead of heating the water from cold, you will instead be topping off an already warmed cylinder.
It’s like re-boiling a half-boiled kettle.
During the winter, your Solar Panels will still provide a degree of preheating for your storage tank. Most likely they will heat the tank to about 20 degrees – you will normally shower in water of around 37 degrees C. The rest of the heating can be done reliably by your existing central heating which will likely be operating anyway. Furthermore, you will gain additional efficiency from being able to store this heat, as opposed to having to run the immersion each time hot water is needed.
It is possible to oversize the solar panels – specifying extra solar panels above what would be normally required to heat your storage tank – to improve winter performance at the expense of requiring some method of heat-dumping in summer.
Solar Panels are well suited to driving underfloor heating systems which operate at lower temperatures. This will require a larger area of Solar Panels to be installed along with a buffer vessel to act as a store of heat, but for well insulated houses or for bathrooms there can be achievable benefits. For radiators, solar panels will not normally provide the required temperatures. The simple fact is that the demand from a space-heating system is at its highest when the energy available from the sun is at it’s lowest i.e in winter.
It’s payback time.
The Payback period from your solar panels will depend on multiple factors. It depends on how you use the hot water, how you operate the system, and how you currently heat your hot water. Hotfoot are capable of giving an estimate of the annual yield from your solar panels using DEAP and TSol software. This will measure your yield in kilowatt-hours – a figure translatable to electricity input through your immersion heater. Energy supplied by your solar panels is energy that you do not have to purchase.
One must also remember that, over the last ten years the cost of electricity has doubled, while the cost of oil has more than trebled. There is no reason not to expect that this trend will continue into the future. All it will take is a 7.5% annual increase for energy prices to double in ten years time. The more expensive oil, gas and electricity become, the better your savings will be.
Hotfoot estimate that the majority of domestic solar panels installations will achieve payback within a range of 7-10 years. It’s impossible to be more exact than that. Payback periods of 5 years or less are possible on certain systems, especially for smaller homes with SEAI grant-aided installation.
However, solar panels in Ireland can easily last up to 30 years with proper maintenance so will pay for themselves almost three times over in their lifetime – there are not many investments out there capable of that return!