Homemade renewable heat
Solar thermal systems for domestic hot water in residential buildings are a common application worldwide and represent the largest share of the solar thermal technology market. For residential buildings, the solar thermal technology is rather mature and has been present in the market for over 30 years. In terms of applications, it can be subdivided into the supply of domestic hot water, space heating (or combi, when combining both), cooling or low temperature for swimming pools.
There are two main systems used in residential buildings: thermosiphon and forced circulation. Thermosiphon systems are simple and compact, including both the collectors and the thermal storage in one unit placed on the rooftop. As such, they are especially effective in warmer climates. Forced circulation systems use a pump to circulate water between the collector on the rooftop and the thermal storage inside the building. Therefore, this is a common and effective solution in colder climates.
Description of use
Half of the energy consumed in Europe relates to heating and cooling. In residential buildings, the amount of energy needed for space heating and water heating accounts for 80%. Water heating alone represents 15% of the total needs, equalling the electricity consumption in households. Decarbonising heat in homes is thus imperative to reach EU climate targets.
The annual domestic hot water consumption in developed countries is around 1000 kWh per person and rather constant all year round. Solar thermal collectors are therefore particularly suited to meet this demand. Their operating principle is simple and effective: The solar collector captures the energy from the sun, warming up a fluid which is kept in a thermal energy storage unit, ready to be used for space and water heating.
In higher latitudes, where the radiation in winter is significantly lower than in summer, such systems can cover between 60 and 80% of the annual domestic hot water demand. This means that a supplementary or backup heater is required. In low latitudes, solar thermal can cover 100% of the domestic hot water demand, requiring a backup heater only for exceptional conditions.
Benefits of Solar Thermal
Systems!
The benefits of solar thermal systems, cover environmental, political and economic aspects.
Environmental benefits relate to the capacity to reduce harmful emissions, impacting both our environment and our health. The reduction of CO2 emissions depends on the quantity of fossil fuels replaced directly or indirectly by the solar thermal system, for instance gas or carbon-based electricity used for water heating. Depending on the location, a 2.8kWth (4 m2) system could generate the equivalent of 2.2MWhth/year, a saving of around 350kg of CO2.
Political benefits are associated with the possibility of improving energy security by reducing energy imports while creating local jobs related to the manufacturing, commercialization, installation, and maintenance of solar thermal systems. The solar thermal sector has a strong European manufacturing base, supplying over 90% of the local demand and exporting worldwide. Therefore, opting for solar thermal systems in Europe means choosing solar energy produced by European companies, boosting the European economy and creating green jobs.
Economic benefits are associated with the potential savings in energy costs. Even though solar thermal systems require higher upfront costs, they offer an economic advantage in the longer term. Solar thermal systems ensure price stability for at least 20 years. Analysis of existing systems demonstrates that solar heat is economically competitive with other renewable energy sources and gas. This competitiveness is even more accurate when energy prices fluctuate, as seen during Europe’s recent energy crisis.
There are three main aspects to consider that have a bigger impact on the comparable costs of the energy produced by a solar thermal system. These are the initial costs of the system, the lifetime of the system, and the system’s performance. These factors depend on the location (affecting climate, insulation level, taxes, cost of living, etc.) and quality of the system (affecting performance, lifetime, and cost).
This can vary significantly from country to country, thus leading to different average investment costs for solar thermal systems.
In the Mediterranean region, the average cost for an open-loop, pressure-less thermosiphon systems is around 830 EUR. Pumped indirect systems investment costs can go from 765 and 1710 EUR/kWth in central Europe, while in northern Europe they can go up to between 1440 to 2160 EUR/kWth. In terms of energy costs, the former can range from 9.5 to 13.5 EUR cents/kWhth while the later may range between 16.7 to 26.55 EUR cents/kWhth. The most competitive systems in Europe are the thermosiphon systems in Southern Europe, with costs as low as 4 to 6 EUR cents/kWhth.
Explore Solar Heat for Residential Buildings
Case Studies
Technical information
The operating principle is rather simple:
The sun heats a fluid in a solar collector, which is then used to heat domestic hot water that will be stored in a tank, ready to be used. The system consists of solar thermal collectors, pipes and a hot water store. A thermosiphon system, the simplest, may use only these components. Forced circulation systems are more complex and require, besides the basic components mentioned above, pumps, controller, heat exchanger, valves and backup heater.
The solar thermal collector is the main component of the system. Within the collector, the solar irradiation is captured by an absorber and converted into heat. To increase efficiency, the absorber is otien selectively coated, which means that the absorption of the irradiation is maximised, but the emission of heat is minimized. The absorber heats a fluid circulating in contact with it. This fluid can be just water, a mix of water and glycol (to avoid freezing during the winter in colder climates) or another heat transfer fluid. Solar thermal collectors an heat directly the water that will be used: these are called direct systems. In such systems the water for domestic use circulates inside the solar thermal collector.
In contrast to direct systems, indirect systems use two circulation loops. A first closed-loop system allows the circulation of the heat transfer fluid between the collector and the heat exchanger. A secondary loop is then used for the circulation of the water for domestic consumption between the heat exchanger and the hot water storage. The heat transfer between the solar fluid and the water is done by means of a heat exchanger.
One common example of an indirect system are thermosiphon systems. It takes the name from a physical effect that uses natural convection generated by the expansion of a warm liquid, becoming less dense than colder liquid. The warmer liquid hence moves upwards while the colder liquid moves downwards. Therefore, the thermosiphon systems operate without a pump, using natural convection to carry the hot water to a storage tank located above the solar collector. Because the storage tank is located on the roof, gravity is used to bring the water from the tank to the house.
Another example of an indirect system are forced circulation systems. In this case, the hot water storage tank is located inside the house. This is a common solution in colder climates, to avoid heat losses in the tank during the winter. In these systems, the water is pumped between the collector and the hot water storage tank.Additional backup heating systems, such as gas boilers, usually also feed into the storage tank to cover the eventual shortcomings of the solar thermal system, especially in winter.
Temperature: Between 40 to 60 degrees Celsius.
This is the usual temperature range required for the most common uses, even if the user
lowers the temperature by mixing with cold water.
Control: Simple metering and simple control.
These systems require simple metering and control. It is possible to have more advanced
metering and control, and even remote monitoring, but the costs are relatively high when
considering the overall investment in such systems. In forced circulation systems usually the
control is done using a “solar station”, comprising in one device several components
controlling the system.
Operation & Maintenance: Generally very low, higher in colder climates.
The operating and maintenance requirements are rather simple, requiring usually one
routine visit per year.
