The Solar Heating Group at DTU is working on a method for storing solar energy. The key component is a giant container of salt.
The replacement for the old-fashioned, eco-unfriendly oil-fired boiler is now being installed at DTU in Lyngby. Researchers from DTU Civil Engineering are developing a system designed to solve one of the key problems associated with the otherwise extremely sustainable solar power: how to store solar energy for the cold, dark months.
The problem is straightforward, but hard to solve. For obvious reasons, solar heating is more efficient in the summer months, while the need for power is greatest in the winter when solar panels are not generating as much energy. The Solar Heating group at DTU Civil Engineering, headed by Associate Professor Simon Furbo, in consultation with researchers from countries including China, and backed by EU funding, has now come up with a solution for storing the energy from solar generators for the cold months—a breakthrough that has otherwise proved irritatingly elusive.
The heat from the solar panels is stored in a salt hydrate, a solution of sodium acetate that features a number of special properties. For example, when heated to melting point, the salt hydrate can store energy without losing it—which is what tends to happen at present, when the ‘storage solution’ is simply a tank of water that is heated up.
The melted salt retains a large volume of energy which, in principle, can remain there forever because the energy is not released until a crystal is added to the salt. In this way, solar panel owners can put some of the summer sunshine aside for the winter months, as Jakob Berg Johansen, Research Assistant at DTU Civil Engineering explains:
“As soon as the salt reaches a temperature of 58 degrees Celsius, it melts, and the energy we have stored in it stays there. When the salt cools down again, it tries to crystallize. It cannot do so, however, until it has something to form crystals around, which is what actually triggers the process. When we add even a tiny crystal to the solution, the entire volume crystallizes in around a minute—releasing the energy and the heat in the process. It instantly returns to the 58 degrees again, and that is sufficient to heat hot water and homes,” he says.
Jakob Berg Johansen and Simon Furbo have a solution for storing solar energy in the future. Photo: Mikal Schlosser.
Market sure to appear
Thus far, Jakob and his colleagues have built a range of prototypes that have been installed at DTU. The team now plans to build what corresponds to a single family home next year, and establish whether they can use solar energy to heat it all year long.
The purpose is to test the system thoroughly before manufacturing and marketing it to the numerous households in Denmark that do not have access to the district heating network and are therefore dependent on other sources of heat during the winter months.
“Oil and natural gas have dominated the market to date, but the situation is gradually changing because people are becoming increasingly aware of the negative aspects of fossil fuels,” says Jakob Berg Johansen.
“There is still a fair amount of development work to be done before the idea is viable as regards price and at a technological level. But we can see that the market’s there—or that it is sure appear—because people are keen to get rid of their old oil-fired boilers. China is the biggest market in the world for solar heating at the moment, and this is clearly reflected in our project, which currently features five or six Chinese researchers.”
Nilan, the ventilation and heat pump company, has contributed development and production of the storage modules that are to contain the salt solution. Henry Yndgaard, Senior Project Manger at the company, emphasises that the desire to participate in the project rests on two pillars:
“All the modules we have delivered are hand made, and that is a cost-intensive process. However, it is an extremely exciting project, and the overarching theme of storing solar heat contains a lot of potential; we naturally want to be at the cutting edge in this regard—especially taking into account future business opportunities,” he says.
Article in DTUavisen no. 1, January 2015.