Solar Water Heating explained
Many people choose solar for different reasons. They might want to be more self sufficient, save money on their power bill (water heating counts to on average 30%) or simply like the idea of utilising renewable energies. Solar will potentially tick all of those boxes for you. While access to sunlight is dependent on your specific site, roof direction and the mounting angle of the collectors, solar can greatly benefit most New Zealanders and their wallets in the mid to long term. Solar systems come in different shapes and forms. Here, we want to explain them for you to make a choice that is right for your circumstances, whether renovating or building new. There are quite a few options in collectors and systems, so please bear with us to get most of it out for yourself.
Solar water heating has come out of fashion in New Zealand. The past NZ1,000 grant from EECA attracted many companies into the industry that did not necessarily bring in the best product and once they disappeared many solar customers struggled to find help for failing pumps and controllers. However, solar is a good alternative for many who have a north facing roof and like to utilise free energy. Many good companies are still in the market ensuring you will get a system that lasts and works for you.
Find out with our free calculator how much solar or heat pump water heating could save you per year.
Solar hot water heating does in New Zealand currently require a building permit to be installed. This does add to the cost of solar and increase the payback time. In most locations around New Zealand, a standard solar system has a payback time is between 5-9 years. Solar does add value to your home and will last in many cases 30 years or more, often saving the owner above $20,000 or more in the process. The key to success in solar is proper design that fits your property and requirements.
Types of Solar Collector:
Flat plate collector
The flat plate collector is a flat plate that has multiple layers: at the bottom is insulation to ensure heat is kept in the water pipes above. Above the insulation is a thin reflective film/ plate, that reflects the sunlight into the pipes to utilise as much energy as possible. The pipes where the water is carried are above the reflective film/ plate. Water stays in the pipes until it hits a certain temperature and is then pumped into the water tank. A safety glass on top which also helps to protect the pipes from the colder temperature is installed on top. The safety glass is as well often shaped to let sunlight in but does not allow all of the sun rays to escape back into the atmosphere; thereby creating higher temperature in the water pipes.
Flat plate collectors are the most affordable version of solar panels, however they are also the most inefficient due to the heat loss of the insulation. To get the same amount of heat, a bigger roof area for the solar panels is required. Though most of the time 2 panels are enough. Flat plate collectors do have the advantage that they are more resistant during hail. My past experience in the field had fewer insurance claims and replacement requirements for flat plates than evacuated tube collectors.
2. Evacuated tube collector
In an evacuated tube collector there are multiple tubes. Within the tubes are pipes with a water and glycol mix, which prevents it from boiling in high temperatures. The glass tube around the pipes are in a vacuum, which is the best insulation known to date. When the water/ glycol mix heats up, it rises. At the top of the panel runs a pipe with water through. A heat exchanger transfers the heat of the water/ glycol pipe to the water pipe which then gets pumped down into the water cylinder. The vacuum tubes and the panel behind are all round to reflect and capture the maximum amount of heat within the pipes.
Evacuated tubes are more efficient than flat plate collectors, however are also more expensive. The vacuum of the evacuated tube makes it more efficient, however if there is a small crack in a tube the efficiency drops dramatically and this can go unnoticed.
3. Thermodynamic solar collector
A thermodynamic solar collector is different. It basically functions like a heat pump. To heat the water it utilises the warm air around the collector and uses the warm air to heat a refrigerant liquid. Why not just use a heat pump? The air temperature especially on metal roofs is higher, increasing the efficiency of the heat pump technology. From the panel the heated refrigerant liquid is pumped into a compressor which heats the liquid further. From there it is pumped into a heat exchanger that is within the water tank, heating up your shower or tap water. On giving out its heat to the water the gas cools down and becomes a liquid again, ready to be pumped back into the panel. From there the circle begins again.
This is a relatively new and innovative type of collector. Important to note is that the temperature for it to work needs to be above -10 degrees Celsius, which is in the vast majority of New Zealand no problem.
Types of Solar Water Heating System
There are two different types of solar hot water systems- they are called open and closed loop system. The difference in between open and closed loop systems is quite simple: open loop means that the water coming out of your tap is going directly through your solar panel. In a closed loop system there is a water/ glycol mix that goes from the solar panel into a heat exchanger in your water tank to transfer the heat into the water. There are also different systems within open and closed loop which we explore at the end; the drainback and thermosyphon system. Both systems have pros and cons.
The closed loop system
The advantages of the closed loop system is that the water glycol mix doesn't freeze and has a much higher boiling point. In real terms this means that there is only little potential of the panels freezing and that the solar panels still perform heating your water tank at higher temperature in the panel. Also, once the hot water tank hits its temperature and the solar turns off, the higher boiling point means less potential damage to the water tank or the solar panel.
The disadvantage of the water glycol mix is that it is slightly less efficient, due to the fact that in the heat exchanger not all of the heat gets transferred as in an open loop system.
The open loop system
As said, in the open loop system the water of the water tank goes directly through the solar panel. This means that it is slightly more efficient than the closed loop system until the hot water cylinder has hit its main temperature.
In high temperature water produces steam as the mass expands. To ensure that the pipes do not burst from the pressure created by the expanding mass of water and steam a valve on the top of the panels is installed. This valve releases some of the pressure and keeps the pressure at a manageable level for the solar panel.
An important thing in an open loop system is to think about is how to deal with freezing temperatures. The controller will continually measure the temperature in the solar panel. If it falls to 4-6 degrees Celsius (can be adjusted) then the pump will kick in and pump warm water from the water tank up to the solar panel to ensure it will not freeze.
The drainback system
In a drainback system there is a heat exchanger in the water tank. Instead of a water glycol mix in the solar panel there is distilled water. When there is heat on the panel, then the pump will push the water from the heat exchanger to the solar panel. Once the water is heated up, gravity will take it down to the water tank. When there is no heat up in the panels or the water tank is heated up completely, then the pump is shut off and no water can access the solar panels. The advantage of this type of system is that it cannot freeze as there is no water in the pipes to freeze on the roof.
A thermosyphon system is where the hot water tank is situated above the solar panels. This usually means that the cylinder is on the outside on the roof, right above the solar panels. In a thermosyphon system no pump is required. A thermosyphon system can have the water from the water tank or a water glycol mix with a heat exchanger in the cylinder. As the water or water glycol mix heats up in the solar panel, it rises and goes into the water tank or heat exchanger. Once it cools down it drops again thanks to gravity from the water tank or heat exchanger into the solar panel, restarting the heating process.
It is important to check out that your roof structure can support the water tank. An engineers report is often necessary for this. In freezing zones only closed loop systems that have a water/ glycol mix and a heat exchanger to ensure that the pipes in the panels cannot freeze are recommended.