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Solar Thermal Power - a "short" guide for beginners

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  • Solar Thermal Power - a "short" guide for beginners

    Before you get started, research what options you have for installing solar panels, and how much time and how many funds you are prepared to spend on it.

    Solar energy, whether it is electric or thermal, is an investment like any other: First, an effort must be made, and then it pays for itself over a period of time. The more return you want, the more you can expect to have to enter into the project. Of course, it depends not only on the money and time spent, but also how smart you make the system.

    Solar Collectors: Flat or Vacuum Tubes?

    Both types are really good, but if you want space heating, then it is probably a good idea to consider vacuum tube solar collectors. Especially if you use a type with mirrors that can concentrate the light a little more. This solution is better when it is partly cloudy and cold. If you just want "hot water", then flat solar panels are fine.

    Frost-proof liquid or drainback system?

    In some countries, there is frost in the winter, so either the medium must be frost-proof (usually water mixed with, for example, propylene glycol), or the liquid must be automatically emptied of the system when it is not in use. The latter is called a Drain-back System. The latter has a number of advantages, but it is crucial to get it built properly, because if it does not empty as it should, then there is a great risk of frost damaging the components that are outdoors.

    In addition, make sure to protect against overheating, typically liquid that boils and is thus vented through a pressure relief safety valve. Here the drain-back system is again superior as the water simply drains back if it gets too hot.

    Heat storage - hot water tank, radiators, underfloor heating ...

    The simplest system contains a hot water tank, typically a minimum of 300 liters. The more heat you can store, the better the system will do at night or when it is cloudy or foggy.

    You can connect it with a radiator system, but the best heating system for solar heating is definitely underfloor heating. As much as possible. Underfloor heating lasts a really long time, and it requires a lower temperature than e.g. radiators. In some climates, you can even use underfloor heating for cooling in the summer, by letting the floor send heat into the solar panels at night, where it is then radiated out. If you want even more out of it, then you can build radiant heat into walls, fireplaces, and other structures with mass in the house. The more mass and surface you can heat up, the longer it lasts, the lower the temperature needed, and the more energy you can suck out of the solar panels in the time they can provide something.

    You can also build external heat storage, in water, sand, soil or whatever else you can find of mass that can be heated. Or you can put several hot water tanks in series. Or you can use hot water buffer instead of container (see below). Each solution has advantages and disadvantages, and typically a combination is the best solution. You can even slide pipes under the house, and then heat the ground in spring, summer and autumn, and then get a little supplement for the heat in winter via heat that rises (and frost that is kept out). This principle has many names, typically seasonal heat storage, seasonal heat storage etc. There are also some who build a heat storage next to a building, typically dug into the ground.

    Solar thermal for hot and humid climates

    In climates like that in Hong Kong, heating requirements are minor compared to those of air conditioning. It's mainly the moisture content that makes it uncomfortable. Air can be dehumidified using heat in Desiccant Dehumidifiers, while Absorption Chillers can cool the air. These technologies will be essential parts of the PleasAir system, developed by Ciao Carbon limited. As solar thermal is a simpler technology than solar PV (electric), it also captures around 4 times as much energy in the same area, as electric does. You will still need electricity for light, gadgets and other appliances, but heating and cooling doesn't need electricity, except for control system, fans, sensors, pumps and so on. They hardly use any power, and power can easily come from wind or solar energy as required.

    Hot water tank or hot water buffer?

    Instead of sending mains water into the bottom of a tank and extracting it at the top, you can use a hot water buffer instead. This tank uses the same water all the time, to keep the heat energy, typically the same water you send in radiators and underfloor heating. For the domestic hot water, you can then either have a coil in the container where the cold water is heated, or you can have a heat exchanger mounted on the outside of the hot water buffer. Both have the advantage that the hot water is always "fresh". In a traditional hot water tank, the water plummets around and there is a danger of e.g. Legionella infection if you do not keep 60 C in the tank. Solar heat can not always provide 60 degrees, so a hot water buffer. solves that issue. While the water is always replaced in a system where the domestic water is only kept in pipes, there is never any "old water" that can be a place for legionella to grow. In principle, you can use the hot water from a coil for hot drinks and cooking, so instead of having to heat the cold water for coffee or potatoes, you can e.g. start with 60-70 degrees.

    Expansion tank and safety valve

    Water is really good for solar heating as it is non-toxic and easily accessible. Unfortunately, it can both freeze and boil, and this must be taken care of with e.g. a pressure relief safety valve and if required, frost protection. In addition, water expands when it is heated, and therefore you must allow the water more space, e.g. with an expansion vessel. Should this fail or otherwise be unable to keep the pressure under control, the last resort is a the pressure relief safety valve. Without such a valve, there is a risk of violent explosions that can cause not only major property damage, but also personal injury. A well built system will never require any pressure relief safety valves, but it is still there as the last resort.

    If you choose a drain-back system, the water expansion is built into the system, but for closed loop systems it is important to size the expansion tank correctly.

    Solar heating efficiency

    For solar heating, it generally applies that the more you can cool the solar collectors with the liquid that is circulated, the more heat you have produced. And the more heat you have produced, the less there is a need for another heat source, e.g. furnace, district heating, wood stove, etc. It is obvious that in order for the solar heating installation to pay itself back, you must save the energy you are paying for. In addition, there is the environmental aspect that solar heating does not emit anything.

    A common misconception is that solar panels should preferably be as hot as possible in order for them to perform the most. It could not be more wrong! Compare that to a car with an internal combustion engine, where revolutions per minute are compared with the temperature in the solar panels. If the clutch is depressed or the gear box put in neutral, and you step on the accelerator, then you can get a lot of revolutions per minute. But nothing useful is provided because it does not move the car. The same for solar heating: If you stop circulating liquid, the solar collectors will get really hot - but they do not provide anything useful. As in the example with the car, there is a spot in between hot and cold which is optimal, and that is where you extract the most heat. It is simply the difference in temperature in and out of the solar collector, multiplied by the flow that is, for the heating density of the medium in question (eg water).

    Therefore, the more you cool your solar panels, the more energy you get out of them. Or in other words, the hotter the solar panels, the more energy is wasted to the surroundings.

    However, there IS an exception to this "rule", and that is that in some cases one will e.g. rather have hot water, than heating. In these cases, you sometimes have to compromise and let the pump run slower. Thereby the temperature in the solar collectors will rise, e.g. up to a temperature where they get sufficiently hot to use for hot domestic water. This means a lower overall performance, but it can be a necessary sacrifice to get the temperature you are looking for.

    Control system and electronics

    You can install a complete passive solar heating system, without pumps, controls, etc. Typically a container on the roof, where the temperature of the solar collector auto-circulates the liquid (typically water). These systems should be emptied before the frost sets in, if you are in that type of climate, and can typically be used in summer vacation houses or elsewhere, where you just want hot domestic water.

    For more complicated systems, electronic control, sensors, pumps, etc., are needed to make it work.

    We are using and suggesting the use of control modules from T.A. - Technische Alternative in Austria. We will be selling the products from T.A., starting some time in 2021, via the Ciao Carbon Limited website.

    You must use at least one control module. If you choose a module with CAN bus, then you can connect several modules together, and more importantly, you can connect it to a box called a CMI. This box can be connected to the Internet, so you can monitor and control the system from a web browser on smartphone / tablet / computer. With graphical schematics, often on several separate pages if it is a more advanced system. you are able to click directly on pumps, valves, etc., to control the system to what you want. You can even program the modules (those that can be programmed) via the Internet. An SMS module can also be connected, so you still have (partial) contact if the internet fails. In addition, CMI provides data logging, so you can see what has happened in the past, measure heat production, etc. It can also send alarms to email (and/or SMS, if you have that module), if something should go unexpected in your system.

    In addition to control modules, you must of course have sensors, pumps, valves, etc. The more you want out of the system, the more you must expect to invest in it.

    Software for T.A. the modules can be downloaded for free here. Even if you choose control boxes other than those from T.A. you can still use their TAPPS2 program, via the "Hydraulics" tab, to design your own system. It requires Windows or LINUX, or if you have Mac OSX, via an emulator such as VMWare, Parallels, etc.

    Programming and T.A. software in general, will be explained later in the relevant forum.

    Feel free to ask more until what may have been forgotten above, and this guide will be updated.
    Last edited by Core; 02-27-2021, 04:25 AM.

  • #2
    Is this a public document? I ask because it contains some technical accuracies.


    • #3
      Originally posted by johnherbert View Post
      Is this a public document? I ask because it contains some technical accuracies.
      Yes it is public.

      I presume you meant inaccuracies?

      Please feel free to let me know what is wrong, then I will correct it.