Energy storage: Charge your battery first, then make hot water

10/11/17, 11:01 AM -

To increase self-consumption, some manufacturers offer special heating elements for heating water. The control units can be integrated into a smart home system. This in effect creates hybrid energy storage, consisting of a solar battery and a heat collector.

Solar expert Martin Hackl of Fronius shows the new Ohmpilot.
Solar expert Martin Hackl of Fronius shows the new Ohmpilot.

Within the span of just a few years, integrated energy is being used to heat drinking water. By now – at least in new buildings – it has even become the norm. Either, the solar power is used to generate hot drinking water, or, via a single- or three-phase heating element, it is directly input into the water buffer. Obviously, fuel cells will also soon use this technique to use electricity to make heat.

The end of classic combustion-based technologies is in the offing, and is approaching fast. Only in the area of renovation do gas- and oil-fired furnaces still have a certain grace period.

Effort expended for hot water is declining

This can only benefit customers: The overall technological effort expended to generate hot water in an energy-efficient and hygienic manner is declining massively.

And it is mostly the demand for hot water that is posing challenges for building planners. Expectations in Germany are high, the customers of planners and architects are used to getting the best: On average, a household uses 120 litres – per day! About a third of that is heated, commonly referred to as either hot water or raw water.

45 degrees Celsius are enough for the kitchen

In former days, the supply of heating was linked to the production of hot water: The gas-fired boiler would also heat the storage tank for hot drinking water. The result: For most of the year, the boiler would run only at low partial load and would cut in often, causing wear and tear, higher maintenance costs and increasing emissions. And: As everyone knows, gas-fired technologies are dangerous, liable to explode. Furthermore, they have always resulted in unpleasant smells and deficient indoor air quality.

Producing hot water electrically, however, is much more efficient. No longer is it necessary to generate almost 1,000 degrees in a furnace, only to bring drinking water up to 50 degrees. More is never really needed, because even 45 degrees will suffice to clear away even the toughest kitchen grease.

Varied possibilities for storing

Using electricity makes controlling the heating of water much easier and more efficient. Once the water reservoir has reached a sufficient temperature, the heating element switches off and surplus energy is either used by electric consumers in the house, stored in the solar battery or fed back into the grid. The most elegant solution includes an EV – a mobile storage battery on four wheels.

It is conceivable – and actually doable – to reduce the amount being fed into the grid down to zero by raising the temperature in the water reservoir. Incidentally, that also helps to protect against germs (Legionella).

Variable control as standard

The electric heating element, also known as a heating blade, is introduced to the water reservoir through a connecting flange and there converts the surplus solar electricity into heat; just as in an immersion heater.

Single-phase heating elements usually have a capacity of up to three kilowatts, three-phase models go up to nine kilowatts. These days, variable control is a standard feature, but there are also products where the heating output is controlled in stages. The heating element is commonly placed at the top of the reservoir, in order to reach high temperatures as quickly as possible. Or to get the temperature up to 70 degrees Celsius to protect against Legionella.

Some municipal utilities have taken to integrating gigantic heating elements of several megawatts into their distributed heating systems. Not strictly on-topic, but it shows that generally speaking there is no upward limit.

70 degrees Celsius to fight Legionella

The heating elements are very efficient. Practically the entire solar electricity is converted to heat energy. However, there is one obvious disadvantage: Heating elements require a water reservoir to transfer the heat to the drinking water. And such tanks are bodies of stagnant water where germs can accumulate. That is why all the relevant norms mandate that the reservoirs should be heated up to over 70 degrees to eliminate germs and microbes.

And that, in turn, means that for reasons of hygiene, more energy has to be expended than is strictly necessary. At the faucet, 44 to 45 degrees, or even not less, are perfectly sufficient. For a bath or a shower, it can be as low as 35 degrees.

To get from 70 degrees in the water reservoir down to 44 or 35 degrees requires the water system to add cold water. This means that not only energy but also water is wasted. Furthermore, there is the risk of scalding in case one of the children accidently opens up the tap that has near-boiling water coming out.


Be aware of calcification

One benefit: Electric heating elements are mostly maintenance-free. The drawback: As time passes, like the rest of the installation and the reservoir, they are subject to calcification. This detrimentally effects the energy throughput of the heating element. In effect, it acts like a form of insulation. Thus in some areas with highly calcified water it can be advisable to decalcify that water by using filter cartridges.

What is crucial is how the heating elements are controlled: This can happen by way of a floating contact at the inverter, which switches the heating element on or off as needed. More modern systems are integrated into the energy management system through a control box, or even as part of the building’s automation system. It is also possible to control it via a sensor next to the building’s metre. The heating element is activated as soon as the metre registers excess feeding-in.

Always keep hot water and heating separate

Generally speaking, it is important to separate the systems for hot water and heating. Anyway, water used for heating must not get into the drinking water. But a separation also makes sense from an energy viewpoint; mostly for reasons of energy efficiency and in view of the operating costs.

Two separate systems for hot water and heating can be planned according to the requirements of each and be adapted to the individual demands of the customer. Again, electricity has the advantage that it can be better planned, installed and managed. (HS/HCN)

Read more about energy storage

The great vision of Fronius: 24 hours of sun

Stay informed, get our free newsletter twice a week, register here

More useful information: