Take heat from the surroundings, add a dash of electricity, and combine the ingredients in a heat pump. This is the recipe for providing heat and hot water to houses without generating CO2.
A heat pump like this one generates enough heat for several buildings at Siemens.
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Energy sources can sometimes be found slumbering in the most unexpected places. Just below the surface of the Earth, for instance. There, at a depth of ten meters or more, a relatively constant temperature prevails year round. In Germany, this temperature is around ten degrees Celsius — an enormous reservoir of heat energy just waiting to be tapped.
How can a starting temperature of just ten degrees Celsius be used to heat a house to 20 degrees? That’s where heat pumps come into play. Circulating within a heat pump is a medium with a very low boiling point. This is generally a fluorinated hydrocarbon that evaporates between -47 and -26 degrees Celsius and extracts energy from a heat reservoir in the process. A compressor compresses the gas, thus heating it to a substantially higher temperature. Essentially, it “pumps” the heat energy from the ground to a higher level.
The gas is now so hot that it can transfer energy to a building’s heating system or hot water system via a heat exchanger. The gas cools and condenses as this happens. The resulting liquid is then forced through an expansion valve to reduce the pressure before flowing back to the heat reservoir. Here, it evaporates again and the cycle begins anew. The heat reservoir — in this case the ground — has given up energy through all of this activity; but it is so large that there is no noticeable decrease in temperature.
Heat pumps are economically and environmentally attractive because they require only a small amount of energy to drive a compressor. However, thanks to the input from the ground, they generate a large amount of heat. Theoretically, heat pumps can deliver more than four kilowatt-hours (kWh) of heat from just one kWh of electrical energy. How well a unit works in practice is indicated by its annual coefficient of performance (COP). This is the ratio of heat gained to the electricity used over one year. The higher the COP, the more efficient the system.
“The temperature of the heat reservoir should be high and the inlet temperature of the heating system — the temperature at which the hot water flows into the system — should be as low as possible,” explains Reinhard Imhasly of Siemens Building Technologies in Zug, Switzerland. “Because underfloor heating systems require an inlet temperature of just 35 degrees Celsius, they are better suited for heat pumps than conventional radiators, which today still need at least 50 degrees.”
A low inlet temperature also requires good insulation, however, which is why heat pumps are particularly effective in modern low-energy and passive houses. It’s no wonder, then, that the market share of such systems in new housing in Germany has increased substantially from less than one percent in 2000 to around 23 percent in 2010. Heat pumps can also be used in older buildings if these are thoroughly modernized first. “However, it doesn’t make any sense to simply replace an oil heating system with a heat pump while leaving everything else as it is,” warns Imhasly. The market share of heat pumps in renovation projects is six percent. Last year, some 51,000 heat pumps were installed in Germany, pushing their total to about 400,000.
Proper Sizing. “Demand-based control of the heating system using temperature sensors in every room, for instance, is also important. Siemens offers the right products for all types of buildings,” says Imhasly. A key consideration during planning is to ensure that the unit is properly sized. If it is underpowered, the user must also heat by some other means. But if its output is too high, the heat pump constantly switches on and off, which reduces efficiency. The best models allow their output to be adjusted to match heating demand.
The key to achieving an ideal COP is the heat source. Heat pumps can not only extract heat from the ground, but also from groundwater and air. Ground source heat pumps use either a vertical loop — at a depth of 100 meters on average — or a horizontal loop, which is laid one-and-a-half meters below the surface. Water source heat pumps use the heat of the groundwater, whereas air source heat pumps draw their energy from ambient air. Air source heat pumps are easy to install, since they require only a heat exchanger for ambient air. This makes them inexpensive, but also less effective because the outside temperature can drop severely in winter. Groundwater and ground source heat pumps require larger investments, but also deliver more heat for the same electrical input. “In our studies, ground source heat pumps achieved an average annual COP of 3.9,” reports Marek Miara of the Fraunhofer Institute for Solar Energy Systems in Freiburg, Germany. “Air source heat pumps achieved just 2.9.”
According to the German Environment Agency, heat pumps must have a coefficient of performance of three or higher before they reduce the level of CO2 emissions relative to gas condensing boilers given the current electricity mix in Germany. This effect becomes more pronounced as the share of renewable energies increases. In a study for Germany’s Federal Heat Pump Association (BWP), the Technical University of Munich concluded that by 2030 heat pumps with an annual COP of 3.0 could reduce primary energy consumption by around 40 percent compared with conventional systems such as gas condensing boilers. With an annual COP of 4.5, the expected savings would reach 60 percent.
By running a heat pump, users are not only doing something for the environment, they are also saving cash. Although the investment costs for a heat pump can be several thousand euros higher than those for a conventional gas condensing boiler, the investment is recouped after ten to 20 years. In some cases investors get their money back even faster. Five years ago, Siemens installed two heat pumps at its Munich-Neuperlach research facility. The pumps use heated cooling water with a temperature between 14 and 17 degrees Celsius as an energy source. “The savings enabled us to recover our costs after just one year,” says Thomas Braun of Siemens Real Estate. “Today the two heat pumps deliver one-fourth of the heating needs of 30 buildings in which as many as 10,000 people work.”
To get an idea of the future prospects for the widespread use of heat pumps, you need only look to Switzerland, where heat pumps already have a market share of around 90 percent for new single-family homes. Thanks to a large amount of hydro and nuclear power, the Swiss electricity mix is very low in CO2 with emissions of just 127 grams per kilowatt hour (g/kWh) as compared to 563 g/kWh in Germany. As a result, heat pump heating systems there are already producing heat in a highly efficient manner.