What is solar heating?

Solar heating collectors convert the sun's energy into heat. A solar heating system contains an outdoor unit, which are the collectors. The collectors contain water with antifreeze, which is heated by the sun's rays. The heat then passes to an indoor unit which is a heat exchanger.

The heat exchanger can either be directly linked to the central heating system of the building or connected with a storage tank where the heated water from the solar collector releases its heat to the water in the storage tank. The hot water is stored in the tank for up to 3-4 days and is tapped as needed.

A solar heating system usually covers about 30% of the heat consumption (room heating) and 60% of hot water consumption in an ordinary home. It is important to dimension the panels to suit the specific building needs and - if it's a home - also to family size.

There are two main types of solar panels on the Danish market - flat solar collectors and a vacuum collector (rørsolfanger). Basically, they function in the same way, but there are a few differences.

One can also divide the various solar heating system into three categories – for utility water, heating, and utility water and heating combined.

Installations for water

Solar heating systems for utility water are typically small systems, consisting of 3-6 m2 of solar collectors on the roof and a hot water tank of 200-300 litres of water. This size is typically well suited for a family of 2-4 persons. The system can cover about 60% of the total hot water consumption per year.

Installations for heating the building

If the solar heating system is used to heat the building, the system will need to be about 6 -12 m2. An installation of this size can cover about 30% of the energy consumption needed for heating the building, depending on the size and construction of the building.

Installations for water and space heating

Solar heating systems meant to provide both hot water and heating of the house, are typically 9 -18 m2, with a hot water tank of about 300 litres. Depending on the size of the building, the degree of insulation and heating need, the system can cover about 40% of the building's total energy consumption. A solar heating system is therefore not large enough to function as a storage capacity for hot water. Therefore additional heat supply is needed during hours without sunshine or where heating needs are bigger (e.g. in winter).

Placing solar panels

Often solar heating panels are placed on the roof of a house because there is ample space and rarely shade. But the appearance of the panels also plays a role when deciding their location. For many older houses, it is often aesthetically preferable to place solar panels somewhere else than on the roof. It is possible to submerge solar panels into the roof (in-roof); a flat-plate collector will usually be the preferred model in such a case.

Solar panels can be placed anywhere that is shade-free. The output is biggest if the solar panels face due south, and the slope of the roof is between 30 and 60 degrees. If the ground is flat, you can raise the solar panels using a frame.

How it works

The principle is quite simple. The energy from the sun is collected in the outside solar collectors in which water circulates. The sun heats the water in the collector, and the heated water passes through tubing into a heat exchanger inside the house, supplying either the hot water heater or the radiators. Water is conducted back to the solar collector, where the water is heated again. The water from the collector thus circulates in a closed circuit and utility water does not come into contact with the water in the collector. It is solely the heat that is transferred from one circuit to another (not water). Antifreeze is added to the water in the collector to avoid that it freezes in winter.

Why choose solar 

A solar heating system is an energy efficient and economical supplement to the existing energy source of a building. Importantly, a solar heating system cannot replace an existing heating source, but can reduce the need for natural gas, oil, wood or electricity as a heat source. How much of the energy consumption that can be covered using solar panels partly depends on the usage and size of the solar heating system.

Be aware that solar panels cannot stand alone as a heating source. Depending on the size of the solar heating system, system type and consumption needs, a solar heating system usually covers about 30% heat consumption and 60% hot water consumption in an ordinary home. This is due to a number of factors, primarily the need for heating houses in winter when there is less sun. Solar panels are often combined with an oil, gas, firewood or wood pellet boiler. The advantage of combining solar panels with a main boiler is that in the summer, about 4-5 months a year, the boiler can be switched off because the solar heating system is sufficient as heating source.

Life expectancy of a solar heating system

The life expectancy of a solar heating system is 20 years. However, the hot water tank and auxillary equipment such as pumps have a shorter lifespan, usually 10-20 years. Purchasing a solar heating system will economically be particularly advantageous if it is installed while replacing the old hot water tank, an older boiler, or when converting from electric heating to other forms of heating. This can reduce the price of the system by 50%.