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Overview of the Different Types of Solar Hot Water Systems

Spilt systems and roof-mounted solar hot water systems are the most popular alternative to gas and electric water heating systems. When selecting a clean energy hot water system, there are a number of things to consider.

The split system, while discrete and easier to install, may not be the best option if there is a vast distance between the solar collector on the roof and the tank. Challenges posed include water heat loss through the long length of copper pipes between the solar collector and the tank, and the increased cost due to the longer length of copper piping (however this extra cost is most often very marginal).

While roof mounted system are highly energy efficient, they can weigh over 300 kilograms and must only be mounted on sturdy roofs. Many councils in Australia now require a engineering assessment and certificate be provided for the roof the solar tank is to be mounted on, and heavy lifting equipment can add to the cost of installation.

Weighing up all the options is made a little easier if you understand the different types of clean energy hot water systems available on the Australian market, and the benefits and challenges to each.

The Split Panel Solar Hot Water Collector System
A split system is made up of five key pieces of equipment:

  1. Solar Collectors mounted on the roof which heats the water (these can be either a Flat Panel Solar Collector or Evacuated Tube Solar Collector system)

  2. A water storage tank installed at ground level

  3. A solar controller

  4. A small electric pump to circulate the water between the ground mounted tank and the solar collectors on the roof

  5. An electric or gas booster

Solar Hot Water - Clean Energy Centre
The split system uses an electric pump to circulate the water through the solar collectors .The water is heated by the warm rays from the sun via the solar collector. The water is then returned to the water storage tank, where it stays until it is needed for use, or is re-circulated back to the solar collector on the roof when it cools down to a specific temperature.

A solar controller is especially designed to avoid energy wastage and overheating water in the storage tank. This device determines when the circulation pump should run.

With the water storage tank installed on the ground, split systems have the benefit of minimal visual impact, particularly when the solar collectors are mounted flush to the roof, as opposed to mounted on a tilt stand.

Split systems are a lot easier to install than roof-mounted systems, as heavy lifting equipment is not required to position a tank onto the roof. This can save considerable labour and lifting equipment expense, such as a crane.

Split systems are easier to service and repair the storage tanks due to the ground mounted positioning.


Roof Mounted Solar Hot Water Systems

The roof mounted system comprises solar collectors and a water storage tank, both of which are installed on the roof.

The systems design is based on a principle called ‘thermosyphon’. This simply means cold water is heavier than hot water, and thus cold water will fall and hot water will rise. The cold water in the solar collectors receives the warm heat from the sun and so it rises into the tank. This heated water displaces cold water in the tank which then falls into the solar collectors where the process continues. The hot water is stored in the water storage tank mounted on the roof until it is needed for use in the home. Any unused water returns back to the solar collectors for re-heating.

Solar Hot Water - Clean Energy Centre

Roof mounted systems are often referred to as ‘passive’ solar systems because they rely on the principle of thermosyphoning, rather than an electric pump to move the water through the solar collectors. These systems weighs several hundred kilograms (between 300kg to 500kg), therefore the roof must be able to support this extra weight. Also, if in a cyclone prone region, the system will require increased roof fastening to withstand strong wind conditions.

One of the biggest disadvantage to a roof mounted system, and the most common factor to lead people to prefer and choose a split-system, is a crane is often required to lift the heavy tank and other components onto the roof. Craneage services do not come cheap. In addition, many councils now require engineering assessments and certificates to certifiy the roof can support the weight of a storage tank. The process of local council applications adds considerable inconvenience and expense (around $300 to $500).

The Heat Pump

Heat Pump

A heat pump works by drawing heat out of the outside air and converting it to the liquid in the tank via a heat exchange system. A  heat pump hot water system is cery similar to a reverse cycle airconditioner. Some models of heat pumps can be very noisy (like a reverse cycle air-conditioner) and therefore should not be installed close to bedrooms and living rooms as they will run at all hours of the day and night.

 

In most cases, heat pumps are easier to install than a solar system, which provides the benefit of lower installation costs. However, many of the top brand heat pumps are around the same price as top brand solar hot water systems, therefore only marginal overall price benefits are gained, and these are predominantly derived through the lower installation costs.

 


There are many claims that heat pumps are better than solar hot water systems, which could be the case in areas where the ambient air temperature and humidity is consistently high, such as in Northern Australia. However, in mid and southern states, this is not the case and there have been instances where consumers have actually reported an increase in their electricity bills after a heat pump has been installed.

Heat Pumps are required to have electricity supply 24 hours per day and the power supply should not be turned off as this can affect the electronic control units. For this reason, heat pumps are required to be connected to peak electricity (as opposed to off-peak power), which can add 100 to 200 per cent to the cost of every kilowatt used in heating water.

For example, the off-peak tariffs in Queensland include Tariff 31 (11pm to 7am) which costs 8.5 cents per kilowatt (2010/2011), Tariff 33 (18 hour supply per day) costs 12.5 cents per kilowatt, Tariff 11 (peak supply) costs 21.35 cents per kilowatt. Therefore, while you are using up to 60 per cent less energy to heat your water, you may be paying as much as 200 per cent per kilowatt if you previously had an electric hot water system on the off-peak hot water tariff (such as Tariff 31). In such circumstances, you will not save as much money compared to using a standard electric storage tank, and definitely will not save as much money compared to a solar hot water system.

 

Important Note About Heat Pumps...


The Clean Energy Centre will generally recommend a heat pump if the installed roof area is heavily shaded throughout the day, which will impact the performance of a solar hot water system. However, Evacuated Tube Systems deliver good performance in shaded conditions, and hence a heat pump will generally be the last resort recommendation.

 

 

 

 

Considerations for the installation of a Solar Hot Water system


  1.  In the majority of instances, it is inefficient to install a SWH system with no electrical or gas or other fuel backup. Many Solar Hot Water systems have an integrated electrical heater in the water storage tank.
  2. The installation of a SWH system needs to be complemented with efficient insulation of all the water pipes connecting the collector and the water storage tank, as well as the storage tank (or "geyser") and the most important warm water outlets. The installation of efficient lagging significantly reduces the heat loss from the hot water system. The installation of lagging on at least two meters of pipe on the cold water inlet of the storage tank reduces heat loss, as does the installation of a "geyser blanket" around the storage tank (if inside a roof).
  3. Usually a large SWH system is more efficient economically than a small system due to the price of a system not being linearly proportional to the size of the collector. Therefore, a square meter of collector is cheaper in a larger system, making it beneficial to install a system that covers all or the vast majority of the domestic hot water needs. This will facilitate a more rapid cost recovery.
  4. Due to the modularity of an evacuated tube collector panel, this technology allows the adjustment of the collector size by removing some tubes or their heat pipes. Budgeting for a larger than required array of tubes therefore allows for the customisation of collector size to the needs of a particular application, especially in warmer climates.