Solar energy

Solar energy will play a major role in the green energy supply of the future, both locally for individual homeowners and in the form of large power stations, which will help to cover the growing need for green electricity and heating. The annual solar radiation on Denmark’s land is about 180 times greater than Denmark’s total annual energy consumption. This makes the sun an enormous source of energy, and DTU conducts research and teaches how we can make optimal use of it.

Picture of a solar farm on Risø Campus. — Picture of the solar farm on Risø Campus. The farm is two hectares and produces electricity approximately corresponding to the annual consumption of 120 households, when taking into account the benefit of using bifacial solar cells that harvest sun both from the front and the back. Photo: DTU Fotonik.

What is solar energy?

Today, we use solar energy in Denmark in two ways: in the form of rooftop solar panels that can produce heat and district heating, and solar cells that can produce electricity.

Why is solar energy important?

The sun is our largest source of energy, and if we knew how to make optimal use of it, we would easily be able to supply the whole world with energy. Today, only a few per cent of the world’s energy supply comes from solar energy. That number must be increased to achieve the climate goals.

At DTU, researchers are working to make the technologies better and cheaper, so that solar energy can become a crucial part of the clean energy mix that will make us independent of fossil fuels.

How far has Denmark come?

Denmark has come far as regards research and development in solar energy. At DTU, we work closely with the solar industry, and we research, among other things, solar heating systems and integration in buildings, optimization of energy systems, energy storage, sustainable materials for harvesting solar energy, and development of new types of solar cells.

At the moment, the challenges in terms of solar energy are that the sun only shines in the day and the number of hours of sunshine varies. Therefore, DTU researchers are working on finding efficient and cheap methods to store solar energy until we need it. Another challenge is that the solar cells we use today to produce electricity are relatively expensive in relation to their efficiency. They typically only convert 15-20% of the solar energy into electricity. Therefore, there is a need for solar cells that are both more efficient and cheaper to manufacture.

Solar cell module manufacturing is cheap in the Far East, but the modules’ share of the total investment costs is decreasing, and there are many business opportunities for Danish companies in the area of operation and installation of solar farms.

DTU therefore collaborates with the company European Energy among others to develop efficient and cost-optimized photovoltaic systems. This takes place at a test facility at DTU Campus on Risø, where so-called bifacial photovoltaic cells and modules are tested, which can harvest light energy from both sides, in contrast to the more widely used monofacial solar cells. There are no similar test facilities for bifacial solar cells in Europe. The goal is to gain knowledge about the higher capacity as well as any operational challenges associated with the use of bifacial solar cells.

Bifacial solar cells is seen as a promising technology in the solar industry. The hope is that it will be possible to achieve a greater effect for the same installation costs and thus reduce the price of electricity in future.

The solar panels are mounted on tracker systems that follow the sun throughout the day, which allows more energy to be harvested and energy production to be equalized so that it is levelled out during the day. The solar farm on Risø will also use intelligently designed reflectors and study how the entire system should be designed to increase the energy harvest as much as possible—preferably by 30-50%.

The solar farm on Risø Campus has a capacity of 0.43 MWp.

Sustainable and efficient solar cells

Another area DTU’s researchers are studying is how to colour the solar cell modules without compromising performance. Energy consumption in buildings accounts for nearly 40% of the total energy consumption in Denmark, and integration of solar cells in building materials is therefore increasingly important to achieving the goal of becoming independent of fossil fuels.

In addition to efficient solar cell modules, DTU is conducting researching into developing new types of sustainable materials for the solar cells themselves. The majority of standard solar cells today are made from silicon, because this element is found in sand and is inexpensive, sustainable, and able to achieve high energy efficiency, meaning that it can absorb and transform many of the sun’s rays into power. The researchers at DTU have added a thin-film solar cell on top of this technology, a so-called tandem solar cell that can increase the absorption of solar rays and thus the power output. The researchers are now working on refining this technology.

DTU also researches solar cells made of plastic (polymers). Compared to conventional solar cells made of silicon, this type of solar cell requires a much smaller resource and energy consumption during production, and the cells can be produced on a large scale using cheap production methods known from e.g. the graphics industry.

For plastic solar cells to become commercially competitive, their efficiency (i.e. the percentage of the solar energy they can convert into electricity) and their durability need to be improved.

Solar heating

When it comes to solar heating systems, Denmark is the world leader. In fact, Denmark houses the world’s largest solar heating system to date, which was built in Silkeborg in 2016. It can cover 20% of the annual heat consumption in Silkeborg, corresponding to 4,400 households. At the end of 2017, there were 296 solar heating plants (solar heating plants with solar panel areas over 500 square metres) in operation worldwide, 111 of which were located in Denmark.

Solar heating covers approx. 2% of Denmark’s district heating production.

One of the big challenges of solar heating is energy storage, because the sun provides the most energy in the summer, when we need it the least to heat buildings. Heat storage is therefore a major research area at DTU. Among other things, research is being done on a technology that involves long-term storage of energy in salt batteries containing supercooled sodium acetate trihydrate. This principle is known from the small plastic bags of liquid salt that can be used to keep fingers or toes warm in cold weather. When you press a metal disc and release a crystal from its surface, the liquid crystallizes and during this process it heats up.

Another technology involves storing heat in thermal heat storage pits. A thermal heat storage pit is a large, enclosed pit that is insulated and filled with water, which can hold the heat until it is needed. DTU is researching how thermal heat storage pits can be developed so that they become more reliable, affordable and long-lasting.

The pit storage technology is not only advantageous for storing solar heating, but also suitable for all types of district heating systems. A thermal heat storage pit is currently under construction in Høje Tåstrup, Denmark. The pit will be used for short-term storage and will contribute to district heating in the Copenhagen area becoming cheaper, more flexible and more sustainable. The storage pit holds 70,000 m³ and has a charging and discharging capacity of 30 MW and a storage capacity of 3,300 MWh.

Another area of research involves further developing solar panels so they become better and cheaper to produce. Heliac ApS has developed a new type of solar panel with a plastic foil lens that is glued to a glass plate. It acts as a magnifying glass that collects and concentrates the sun’s rays. In this way, the solar panels generate more heat than normal solar panels at high temperature levels, which improves the chances that the panels can be developed more cheaply in the future.

What are the prospects for solar energy?

Recent years have been sunny in Denmark, and electricity production from the sun has been increasing since 2011. So there is potential to significantly increase the energy derived from solar cells, but their full potential is far from being realized, as today solar cells only supply 4% of the power in our energy network.

DTU has calculated the potential of solar cells in Denmark. If private and industrial roofs and fields were fully utilized, it could cover up to 70% of the annual electricity production. However, it is unlikely that solar cells will be installed in all potential locations, but they will certainly be able to cover a larger part of our future electricity consumption, not least because solar cells are constantly getting better and more efficient.

Today, researchers are working on setting up more solar cells in Denmark and finding the right combination with other renewable energy sources while using the energy smartly. According to the Danish Energy Agency’s 2020 Baseline Projection (danish only), solar cells will account for around 15% of Denmark’s electricity production by 2030.

And according to figures from the International Energy Agency, it is expected that solar cells will be able to cover up to 25% of the world’s electricity consumption by 2050.

In terms of solar heating, too, Denmark has good opportunities to increase the capacity for district heating in the coming years. The researchers estimate that solar heating can cover 10% of Denmark’s heating consumption by 2030 and 40% by 2050, if the expansion of solar heating plants with thermal heat storage pits continues to develop as it did between 2012-2019. In this way, the district heating price can to a large extent be made independent of varying electricity and biomass prices.