Warm your house in winter with the summer sun

The solution we’ve all been waiting for: a proposal to store excess renewable energy for use in household heating

By Flavia Bernabo

What would happen if one season’s solar energy could heat apartments and houses throughout the winter, too? Currently, 30% of the energy used in Europe (excluding transport) is used in the household. Of this, over 80% is used for space and water heating. At the same time, 16,000 times more solar energy is received on earth than the total energy consumed annually. An attractive goal, therefore, is to use some of this excess solar energy to satisfy household heating needs. The problem is that solar energy is available unevenly: during the day and not the night; during the summer and not the winter; and not necessarily in the right locations. Ideally, this energy would be stored when it is produced to excess so that it can be used when needed, by retrieving it at the flick of a switch. The sun’s energy could thus be used at night and during the winter (i.e., when it is not readily available).

Current attempts to solve these problems include the Trombe wall, in which a heavy masonry wall receives the sun’s rays through a glass screen. The sun’s ultra-violet radiation can pass through the glass, but the infrared radiation emitted by the wall cannot, and is reflected back to the wall. The wall thus heats up and air can be passed over it at night to warm the house. A more sophisticated concept is that of the Moroccan solar scheme. In this scheme, focussed sun’s rays are used to melt salt during the day. At night, heat is exchanged between the molten salt and water. The salt releases considerable heat upon changing state back to solid and steam is produced, thus generating power. Such schemes, however, have limited storage capacity and for this reason can return energy for only a limited period (i.e., for hours rather than days) and are thus ineffective after sunless days. To make the use of solar energy for heating effective, a longer storage time is required.

A solution to the storage-time problem has been proposed by Fumey and Weber of the Swiss Federal Laboratories for Materials Science and Technology (Empa). Their approach is based on the principles of exothermic and endothermic chemical reactions in an aqueous sodium hydroxide (NaOH) solution. In an exothermic reaction, heat is released; in an endothermic reaction, heat is absorbed. These reactions can take the form of changes in concentration of a solution. Thus, when water is added to an aqueous solution of NaOH (a common and widely available chemical), heat is released. This process can also be reversed. If water is evaporated from the aqueous solution via heating (e.g., by solar thermal energy), the solution becomes more concentrated and thus stores energy. Benjamin Fumey, one of the most influential researchers in the field, claims that their approach achieves a theoretical efficiency of 80% (based on the ratio of useful energy out to energy in), making it a promising storage mechanism. The concentrated solution can store its energy for an unlimited amount of time, or until it is liberated through the addition of water to the solution. Among the advantages of this method, the solution can be stored at room temperature, its energy density is five times that of water, and it is transportable.

Fumey and Weber have constructed a small-scale plant to achieve storage in this way. A 30% solution of NaOH is directed to flow spirally down around the outside of a pipe that is heated internally by water at 60°C, as from a solar collector, and water vapour is driven off to be condensed and stored. In this way, the solution strength is increased to 50%. In the (exothermic) reverse of this process, the high-strength solution is made to flow spirally down the outside of the pipe in water vapour. As it is hygroscopic (i.e., able to absorb water vapour, due to an attraction to water molecules from the surrounding environment), the solution dilutes and exchanges the exothermically produced heat to the water flowing down the inside of the pipe, which can then be used for domestic heating.

The success of this process depends on a number of factors which are currently being examined. These factors include the optimum ranges of temperature and solution strength (which also affect the energy density of the solution for transportation), the development of the plant for manufacture and, most importantly, the associated costs. Fumey and Weber have already achieved storage efficiencies of 80%, which is compatible with the storage efficiencies of pumped storage schemes. In their current work, they are continuing to optimise the differences in the concentrations of the solutions as well as the input and output water temperatures, with the aim of maximising the storage capability of a given volume of solution and achieving a convenient temperature for household use. The next stage of their research will focus on the design for manufacture of a household unit, based on their results from the pilot plant and taking into account the corrosive nature of NaOH in a domestic situation. This stage will be pursued in collaboration with an industrial partner.