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Produce DHW with solar energy and reduce running costs

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Producing hot water calls for a lot of energy, much of which still comes from burning fossil fuels. However, there are freely available alternatives such as solar thermal energy. This means that at least some of the energy required to produce DHW can be provided by a solar thermal system. That reduces the load on the main heat generator and can result in savings. Find out below how you can reduce your running costs with solar thermal energy, what you need to bear in mind and where photovoltaics (PV) can play a role.

From solar energy to domestic hot water

Solar collectors are an essential component of solar thermal systems for DHW heating, as they absorb the sun's rays and convert them into usable heat. Solar fluid transports the thermal energy through pipes to a DHW cylinder, which stores the hot water and supplies it to the draw-off points in the building –– also at a later time, if required. As the heat transfer takes place within the cylinder, the solar fluid is cooled and then pumped back to the collectors, where the cycle begins again. If there is not enough solar energy to heat the water, the main heat generator –– for example a gas heating system –– steps in.

Domestic hot water heating in three simple steps

Step 1

Vitosol solar collectors for DHW convert solar radiation into heat.

Step 2

Solar fluid transports the heat to the Vitocell DHW cylinder.

Step 3

The cylinder stores DHW to cover demand at a later stage.

A solar thermal system for DHW and central heating backup takes the load off the main heat generator. Find out exactly what's involved in the How solar thermal works guide. 

Professional design for optimum demand coverage  

If planned properly, a solar thermal system can provide up to 60% of the energy required for DHW heating. In the summer months, it can even cover up to 100  % and the main heat generator can often remain switched off. This reduces heating costs and conserves fossil resources. If there is not enough solar energy to heat the water, the control unit activates the second heat generator. However, the size and design of the system must be right to optimise demand coverage. Therefore, it's important to consider these factors:

  • Collector area size

  • Cylinder capacity

How big should solar collectors for DHW be?

In addition to the orientation, pitch and shading of the roof, the type of solar collector is also relevant for the collector area size. A basic distinction can be made between flat-plate and tube collectors. The latter achieve higher yields on the same area and are therefore mainly used where space is limited and conditions are less than ideal. The rule of thumb when designing for a family house with average DHW consumption (approx. 50  litres per person) is:  

  • Flat-plate collectors: 1.0 to 1.5  square metres per person

  • Tube collectors: 0.8 to 1.0  square metres per person   

How big should the DHW cylinder be?

The number of people in the household has a direct influence on DHW demand. When designing a solar thermal system with a DHW cylinder, the number of occupants is therefore important. A cylinder volume of at least 50 litres per person is recommended. If appliances with high DHW consumption are to be included as well, this must also be taken into account during the design stage. Ideally, however, the cylinder volume should be determined using a DHW meter, which allows precise conclusions to be drawn about the actual demand.       

Your Viessmann partner can calculate the ideal size of the solar thermal system for you, taking all relevant factors into account.

Photovoltaics as an alternative to a solar thermal system  

Solar thermal systems are the most common solution for producing DHW using solar energy. In principle, DHW can also be produced using photovoltaics –– but via a more "circuitous route" involving electricity. While solar collectors produce energy for DHW heating directly, the solar cells of PV modules first generate electricity from sunlight. One benefit is that this electricity can be used for more than just DHW heating. The solar power can also supply other consumers in the building –– such as a heat pump. This can even take place some time later if a power storage unit is installed.

Domestic hot water with photovoltaic power

There are various ways to produce DHW with PV power. For example, DHW heating with a domestic hot water heat pump has proven its worth, as has the combination of an instantaneous water heater and DHW cylinder.

  • DHW heat pump: A DHW heat pump such as the Vitocal 262-A utilises energy from the environment for DHW heating. Recirculation air, outdoor air or (in the case of a connection to an air distribution system) extract air is used as the energy source. The energy demand of the electrical components needed for heat recovery can be covered by connecting to a PV system. However, the production of DHW with photovoltaic power is not the only benefit. DHW heat pumps also enable ventilation of the place where they are installed, thereby preventing the formation of mould – in basements, for example.
  • Instantaneous water heater & DHW cylinder: Irrespective of the main heat generator, decentralised DHW heating is ideal, for example, if draw-off points in a house are only rarely used or are far apart (which can result in heat loss in the case of the latter). Electric instantaneous water heaters and small and wall mounted storage water heaters from the Vitotherm range provide a flexible supply of DHW. Connecting to a PV storage unit also allows DHW to be heated when the sun is not shining.          

Intelligently combined technologies

The greater the level of self-consumption of electricity, the more worthwhile a photovoltaic system is. Self-produced solar power usually works out cheaper in the long term than electricity from the public grid, even given the acquisition costs for PV. If the suitable roof area is large enough, a solar thermal system and photovoltaics can also be combined. The One Base platform ensures optimum interaction between all components in Viessmann system solutions. 

Answers to frequently asked questions about solar energy and DHW

The choice of solar collectors depends on various factors. Flat-plate collectors are cheaper, but deliver a lower yield on the same surface area than vacuum tube collectors. As the heat loss from vacuum tube collectors is particularly low due to their design, they offer particular advantages where space is limited, there is partial shading or the roof orientation is less than ideal.       

This is possible under certain conditions. If the washing machine does not have its own DHW connection, we recommend installing a time & temperature controller. This protects your laundry by limiting the water temperature. All you need to do is switch the time & temperature controller on before each wash cycle. For the dishwasher, just make sure that the cold water connection is approved for high temperatures.

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