Typically solar panels perform one of two functions, they are either used to generate hot water for bathing or heating solutions or for the generation of electricity. Both can be used for domestic or commercial use and often both solutions are used on the same building giving the end user the ability to reduce both their heating and electrical utility bills.
Solar panels that generate hot water work differently to the solar panels that generate electricity. Thermal solar panels (solar panels that produce hot water) are usually constructed from aluminium, copper pipe, copper sheeting, toughened glass and high quality insulation. They are typically 2.3meters in height and 1.2metres wide and the main frame of the solar panel is made from aluminium because of its light weight and more importantly because of its corrosion resistant properties. The large surface area of the solar panel that is positioned towards the sun (usually with a surface area of 2.5 square meters) has a toughened glass sheet that allows the suns radiant heat to permeate through the glass heating the internal plumbing system within the solar panel.
The internal plumbing system of the panel is made up of copper micro bore piping (copper pipes with a small internal diameter usually 4mm) that runs up and down the panels in a snake like configuration. This micro bore copper is brazed onto a larger copper manifold which then exits at the top of the solar panel on the top left and right hand sides. One of these pipes is the cold pipe connection where the cool liquid coming from the cylinder enters the solar panel, and the other pipe is the hot pipe connection where the heated liquid leaves the solar panel. This pipe either returns to the cylinder or enters another solar panel which allows the sun another opportunity to heat up the liquid to an even higher temperature. The micro bore copper within the panel is laser welded to a copper sheet that is the same surface area of the glass. This copper sheet is covered in special Tinox coating (this is what gives solar panels their blue-black colour when seen from a distance). The dark colour of the Tinox coating maximises the absorption of heat via the copper sheet, therefore maximising the ability of the solar panel to convert the suns irradiation into heat.
The pump which circulates the liquid through the solar panels is activated and regulated by a solar control system (commonly called a solar controller). The solar controller is usually located beside the hot water cylinder in a position that is easily accessible to the home owner or maintenance engineers on commercial systems. The reason for this is so that the end user can get information on how the system is performing. Recent iterations of the solar controllers have a wi-fi option so the end user can assess the system performance information remotely however this technology is expensive and doesn’t add any efficiency to the performance to the system and is therefore very rarely considered by the end user when purchasing the system.
Solar Panel Components
All the wiring associated with the solar panel system is ran to the controller. These wires include:
Solar Panel Sensor Wire: This is a high temperature resistant wire which is positioned in the solar panels and measures the maximum temperature of the liquid in the solar panels.
Solar Cylinder Sensor Wire (Lower): This sensor wire is positioned in the lower section of the solar hot water cylinder and registers the temperature of the water in the lower section of the cylinder (where the solar panels exclusively heat the cold water where it enters the cylinder)
Solar Cylinder Sensor Wire (Upper): This sensor wire is positioned in the upper section of the solar hot water cylinder and registers the temperature of the water in the upper sections of the cylinder
Solar Pumping Station Power Wire: This is a 3 core wire (positive/negative and earth). Power to run the pump comes via the solar controller. This power is only sent to the solar pumping station when the temperature of the solar panels is warmer than the temperature at the base of the cylinder (Solar Cylinder Sensor Wire (Lower) temperature)
Solar Pumping Station Power regulation wire: This is a 2 core wire that is used in the running of energy efficient pumps. It is used to transmit information from the solar pump to the solar controller, this information is used to regulate the speed and the power usage of solar pump (contained within the solar pumping station).
Solar Controller Power Switch: The solar controller is usually powered through a non-switched fused spur and a 5 amp fuse. A 3 core wire runs from this electrical point to the solar controller. This should be left permanently powered on and only turned off if the system is being serviced or the end user is instructed to do so by their solar panel installation engineer.
Solar Panel Mounting System
Solar panel mounting systems are usually metal components that are the inter-phase between the solar panels and the home-owners roof. They should be designed to be compatible with both the roof and the actual solar panels installed. The reason I have highlighted the word ‘should’ is because there are quite a few systems on the Irish market where the importer took in a cheap solar panel system from Asia and modified the mounting system that came with the solar panels to try and make them suit Irish roof designs.
As I have said for many years a Texan wears a 10 gallon hat for a reason, it’s the sun. Irish roofs are unique to Ireland for one reason……our weather conditions, more accurately would be to say two reasons…..our wind and rain conditions! We have some of heaviest wind and rain conditions in Europe and our homes and roofs reflect this. If you work outside in the construction industry here in Ireland you will know the impact a south-westerly wind can have on a building and over centuries our dwellings have been modified to take this into account.
Most of the big solar panel manufacturing companies can give your installer a mounting system drawing based on information supplied by the installer. Information that determines your solar panel mounting system design include:
- Geographical location
- Height above sea level
- Type of tile or slate
- Type of roof (tiles or slates)
- Thickness and length of rafter
Remember the weakest link of a quality made, correctly specified mounting system is the experience of the solar panel installer!
Antifreeze Liquid in solar panels
Solar panels for heating hot water are usually a ‘sealed system’ (a sealed system means that the solar panels system a self-contained pressurised system within the overall plumbing system). The water in your hot water cylinder is heated by moving the heated liquid in your solar panels from the panels to the hot water cylinder.
Most solar panel systems are sealed systems where the antifreeze solution flows within the solar panels, the hot water cylinder solar coil and the solar panel pipework. The pressure is usually set at around 2 bar (but this pressure set point depends on the solar panel system components and the how they interact with each other). Because this is a sealed system the solar panel liquid (antifreeze solution) will increase in volume as the liquid gets warmer and will decrease in volume when the liquid gets colder. Because of this increase and decrease in the volume of the antifreeze solution, a pressure vessel is required to facilitate these volume fluctuations.
There is a connection point on the solar pumping station for the pressure vessel and it is connected to the solar pumping station using a stainless steel corrugated pipework with an internal diameter of 3/4”.
A hot water cylinder that is compatible with solar panels is required when fitting solar panels. The cylinder must have a dedicated solar coil at the bottom of the cylinder. The pipes that are connected to the solar panels are continued into the hot water cylinder via the pumping station. This allows the antifreeze to flow through the hot water cylinder, indirectly heating the hot water within the cylinder.
Solar Pipe Work
The pipework that carries the heated antifreeze from the solar panels to the cylinder must be made from a high temperature material. The most commonly used type of pipework is either corrugated pipework or copper pipe. Whichever type is used must have a correctly specified insulation covering them. This insulation must be capable of working at temperatures of greater than 150°C and be UV resistant.
Solar Pumping Station
This component is at the heart of the solar panel system. It circulates the heated antifreeze from the solar panels to the hot water cylinder. In recent years, European regulation has introduced new energy efficient pumps for all plumbing applications, this regulation changed for solar pumping stations in 2016. This means that all systems now have an energy efficient pump giving a running cost on the system of <€10 per year. A good solar pumping station will have good individual sub components, like non return valves on both the flow and return of the pumping station, a good quality flow meter and a good quality pump. If all these components are of good quality, the pumping station should have a life expectancy of 15 to 17 years.
The mixer valve, or thermostatic mixer valve, is positioned on the domestic hot water pipe that exits the cylinder to service the taps, showers and baths. It allows the installer to set the solar panel system to heat the water within the cylinder up to 85°C. If the installer does not fit this component, the solar controller can only be programmed to heat the water to 60°C.
How to design a correctly sized solar panel system
If a solar panel system is not designed correctly, the end user/home owner will either end up over-paying for a solar panel system that is too big for their needs or a system that is not big enough for their needs and does not generate enough hot water for their daily requirements.
The first question we always ask our customers is how many people live in their home. This gives a very good indication of the size of the system that is best suited to their needs. The next question is what the age profile is of the people who live in the house While this may seem like a personal question (and it probably is) it is asked so we can give the customer the best advice on the size of their system. For instance, if the answer to the question was ‘we have 3 children all in their early twenties and in college’, well, the chances are they won’t be living in the house in 2 or 3 years time. In this case, we would advise the customer that the system best suited to their long term needs is one suited for a family of 4 or less, not one that is usually installed for 5 or 6 people. I guess what I am trying to say is, a system should satisfy the needs of the long term occupants of the house, not necessarily the needs of the occupants at that moment in time. Remember a good quality solar panel system will still be working after 25 years, so it should be designed for the long term needs of the home owners.
Another example is if you have a family with 3 young children. The hot water usage will increase dramatically as the children begin to play sports and enter their teenage years. Therefore, the best investment would be to fit a system that will provide more hot water than they need, just for the first few years after installation. As the children the get older, the solar panel system will need to generate the true amount of hot water that they need.
A correctly specified system will generate 70% of a homes hot water requirement for free with the remainder coming from your oil, gas, back boiler, immersion or wood pellet system. This makes conventional solar panel systems the technology with the quickest payback when compared with other over sold hot water systems.
Solar panels are capable of being used as a home heating system. However, they are only suited to particular situations, ideally houses with high hot water usage during the summer months and use a back boiler system for the winter months. A solar panel system in conjunction with a thermal store work very well, as the thermal store holds the water until required for heating the radiators or underfloor heating later in the day/evening. To keep the costs of such installations to a cost effective level this is ideally suited to a house that is undergoing significant renovation or a completely new build.
Importance of good quality antifreeze
The liquid within the solar panels (sealed system) is polypropylene glycol solution, commonly known as antifreeze. The antifreeze solution is usually pumped into the whole system at the pumping station. This is done using a solar pumping trolley. The quality of the trolley and the manner in which it is done will determine if the system will air lock in the weeks and months after the system is commissioned.
Like any established product on the market there are numerous manufacturers of solar panel anti-freeze solutions, all with varying degrees of quality. A high quality solar panel antifreeze solution will not suffer from rapid degradation in quality over a short time frame or at high working temperatures. High quality antifreeze is less susceptible to air locking after stagnation.
Stagnation is when the cylinder reaches maximum water temperature from the solar panels (usually 60°C-85°C). The controller registers this maximum temperature and then shuts down the power to the solar pumping station, stopping the circulation of heated antifreeze from the solar panels to the hot water cylinder.
When the solar pumping station shuts down, the antifreeze solution that is in the solar panels at that time remains there and the sun continues to heat the solar panels and the liquid within in them .The solar panels will reach temperatures of 140°C-155°C and if it is an evacuated tube system, it can even reach temperatures as high as 235°C!
When stagnation occurs and the antifreeze in the solar panels is heated above 115°C it turns from a liquid to a gas and continues to heat to higher temperatures. If the antifreeze is of a poor quality it will ‘burn’ and the chemical properties of the antifreeze gradually change over time (a number of years). Some of the most significant chemical property changes are a reduction in the freeze point temperature of the antifreeze and a change in the pH of the antifreeze.
The closer that the freeze point of the antifreeze is to 0°C the more lightly the chance of the solar panels being damaged during extreme cold conditions. The weakened antifreeze will freeze, expanding within the sealed solar panel system, bursting and damaging the copper micro bore piping within the solar panels. Once the air temperature increases (above 0°C), the frozen antifreeze returns to its normal liquid form and leaks out of the burst pipes causing a loss in pressure of the sealed solar panel system. Once this happens the solar panels cannot transfer the heat generated in the solar panels to the hot water cylinder. The only way to repair such an issue is the replacement of the solar panels which can be a costly operation.
Deterioration of the pH level of the antifreeze isn’t as serious as a low antifreeze freeze point. Although, it can still cause long term problems to your solar panels – increased acidity of the antifreeze will corrode the braze joints where the copper micro bore is joined to the copper manifold of the solar panels. If the acidic antifreeze is not changed within a reasonable time frame it could cause a leak at the braze points which will lead to the replacement of the solar panels.
In addition to good quality antifreeze, the correct type of antifreeze for your type solar panels is essential for the long term efficient running of your solar panels, as well as the cost of maintenance of the system over its lifetime. I mentioned earlier that the maximum stagnation temperature of solar panels differs depending on the type of solar panel that you have installed. If you have flat plate solar panels installed you can use an concentrated antifreeze which can be mixed with water until the appropriate freeze temperature is achieved (-30°). If you have an evacuated tube system you should have a premixed or straight antifreeze that is never diluted with water.
The reason that there are two different types of antifreeze is because of the maximum stagnation temperatures. If you were to use the concentrated antifreeze with a water mix on tubes (which many inexperienced installers or DIY’ers do, the system will ‘burn’ and have a rapid deterioration in both freeze point temperature and pH level. The system will also constantly air lock and you will have a drop in efficiency when it is working (poorly), increased system downtime when the system completely airlocks and the cost of re-flooding/removing the airlock from the system. The air locking will continue to happen until all the incompatible antifreeze is removed and the correct type of antifreeze, exclusively, circulates within the solar panel system.