Technology

Can a photovoltaic system be used for heating in winter

Heating with photovoltaics: sense or nonsense?

Solar power, as generated by a solar power system (photovoltaic system, in short: PV system) on the roof, can be consumed (own consumption) or fed into the public grid. The latter is becoming increasingly unattractive for solar power producers, as the state feed-in tariff is constantly falling, as required by law: new systems up to 10 kWp, which were connected to the grid on January 1, 2022, already achieve only 6.83 cents/kWh. This speaks in favor of increasing the proportion of self-consumption of solar power in order to save on electricity costs. The question of whether it makes sense to heat with photovoltaics is rightly being asked more and more frequently – which is answered here.

The Federal Environment Agency (UBA) puts the share of final energy consumption for heating purposes in private households at almost half, noting that the final energy consumption for space heating depends heavily on the weather and is therefore subject to major fluctuations. On the one hand, this information proves the importance of heat in private energy consumption and, on the other hand, the potential for saving energy and the resulting energy costs that can be attributed to heat. The Federal Solar Tax Credit is also reducing the amount of tax credits each year.

Good reasons for heating with photovoltaics

Using self-produced solar power to generate space heating could be a way of saving heating energy and heating costs. The following reasons speak for this:

The photovoltaic system on the roof as a source of electrical energy (electricity) is becoming cheaper and cheaper to buy: The prices for systems have been falling steadily for years.

This also reduces the costs of self-generated solar power. It is significantly cheaper than the usual household electricity that is bought from a network provider.

Modern electric heaters (e-heaters, electric heaters) are very efficient these days: They convert electricity into heat with a very high level of efficiency. This means that a very high percentage of the solar power generated by the photovoltaic system and transported to the electric heater is converted into heat.

At the same time, electricity costs are increasing every year. Grid parity for photovoltaic electricity has long been achieved, while energy experts such as Volker Quaschning expect parity for oil to be reached in a few years.

Tip: The climate balance of self-generated solar power is therefore much better than conventionally generated electricity from power plants. According to Greenpeace, their efficiency is only 35 to 45 percent.

Heating with solar power – the technical possibilities and limits

In theory, it is easy to heat with photovoltaics. To be needed a photovoltaic system as a power generator including a power changer and an electric heater as a power consumer.

Tip: The typical performance profile of a photovoltaic system means that it only makes sense as an energy source for e-heating if it is, for example, surface heating such as underfloor or wall heating. Conventional electric heaters are operated with high power and high amperage.

But the direct operation of an electric heater with solar power is only one way to heat with photovoltaics. In addition, there is the second possibility of connecting the PV system to a buffer tank in order to electrically preheat the service and heating water stored in it, whereby the heating control would throttle the output accordingly. Last but not least, the third possibility would be electric heat pumps, which would be operated with solar power and connected to the conventional heating system in order to reduce its fuel consumption.

Large amounts of energy require large photovoltaic systems

In practice, it looks like this: A not inconsiderable amount of electricity is required to generate the desired room heat. Fingers crossed you need about five to ten times as many kilowatt hours of electricity to heat with electricity as to supply electricity, writes Cornelia Daniel in her comment on the solar energy portal. You can use the solar power calculator to calculate how many solar panels you need to power all the appliances in your home.

Let’s calculate it all! A photovoltaic system with 5 kWP typical for single-family houses – consisting of 20 modules with 250 WP each – produces 5,000 kWh of electricity, provided that one kWp of photovoltaic generates around 1,000 kWh of solar electricity annually. This would cover the needs of a normal 4-person household, which is around 4,000 kWh of electricity. 1,000 kWh remained for heating. Is that enough to heat the household? Cornelia Daniel assumes heat requirement figures of between 20,000 kWh (years of construction up to 2002) and 12,000 kWh (KfW Efficiency House Plus 70).

The 1,000 kWh of solar power that remains for heating after the power supply would only cover one twentieth to one twelfth of the household’s heat requirement (solar coverage). Cornelie Daniel also notes that energy demand and production are anything but synchronous and that the proportions mentioned here are illusory. An annual balance would be compared to completely different weekly, daily and hourly figures.

But even if you disregard this, the sample calculation shows how much, or better: how little, such a photovoltaic system can contribute to heating.

Tip: This is not to say that small PV systems for self-production of electricity are nonsensical. no way! But more is needed to generate electricity and heat for service water (drinking water) plus heating: more photovoltaics, i.e. more modules and therefore larger systems. Just to explain: A system that is supposed to deliver a peak output of 20 kWp “occupies” more than 100 square meters of roof space! And if the local conditions, including the orientation of the roof towards the sun, roof pitch and shading, are suboptimal, even more photovoltaic surface is required.

Storage bridges the time gap between solar power generation and heat consumption

Another hurdle for heating with photovoltaics is the temporal discrepancy between solar power generation (supply) and heat consumption (demand), which has just been briefly mentioned: You have to consider that a photovoltaic system in our part of the world generates less solar power during the heating season (October to April). , than during the sunnier summer months. The yield ratio of summer vs. winter can be expressed as 70 vs. 30 percent. In summer, when hardly any space heating is required, more than twice as much solar power is available as in winter! And that is exactly the problem when solar power is to be used for heating.

The temporal disparity mentioned above is also clear when viewed over a shorter period of time: solar power comes from the roof during the day when the sun is shining. But most consumers are out of the house from morning to afternoon / evening: to learn, study or work. These consumers primarily need heating in the mornings and evenings, when it is still or already dark in winter and solar power is no longer coming from the roof.

A storage device (battery) can literally fill the gaps: in stand-alone systems as well as in grid-connected systems. Deep cycle agm battery stores the energy generated and holds it in reserve until it is needed. The storage system thus helps to increase the proportion of self-consumption, which is typically between 20 and 30 percent for small systems in single-family houses with four or five kWp – a self-consumption rate of 100 percent is definitely achievable with an electricity storage system.

Tip: In practice, only residential buildings that have been newly built to the passive house standard or have been renovated to this standard can now be heated exclusively with photovoltaics and electric heating without electricity storage. These houses have a heating requirement of between 25 and 30 kWh per square meter (m2 and year). Thanks to controlled living space ventilation, an electric heater can cover the residual heat requirement of the house.

How can a storage battery help?

How can a storage battery help(Take a 12v lithium ion battery for example)? From a purely energetic point of view, solar power could be used as such in an electricity storage system or already converted to heat in a heat accumulator, to save the day or summer yield for the evening / night / morning or winter.

The solar power storage in the power storage works like this: The storage control would first determine whether the PV power generated would be needed directly in the household, for example for the operation of permanent power consumers such as a refrigerator. If more electricity were currently being generated than consumed, the excess electricity would charge the solar power storage until it was full. Any additional excess solar power could be fed into the public power grid.

In an appropriately insulated heat storage tank, water would be the storage medium: an electric heating element inside could be operated with solar power and would heat the buffered water.

This means that a solar power storage system would have to be designed in such a way that the household to be supplied could at best draw electricity from the storage system from evening to morning. Would the memory rather empty or would there be an extraordinary peak load in the household in between the demand would have to be covered with mains electricity.

So far so good. If it weren’t for the fact that DIY solar power system are currently still very expensive. We are talking here – depending on the performance of the storage – from 4,000 to 12,000 Dollars for a storage that fits into a typical single-family house. The good news is that market researchers, including those from the World Economic Forum, expect storage prices, like PV system prices, to continue to fall.

Conclusion: heating with photovoltaics

In principle, heating with photovoltaics is feasible. But it only makes sense if it pays off. Due to the high PV module prices and the even higher storage prices, the calculation does not yet add up – but price declines and thus profitability are to be expected here in the near future.

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