Solar thermal refurbishment of apartment buildings gains momentum – on the sunny side

Heating and hot water using a solar thermal system is a clean, ecological and, in the long run, economical thing to do. However, until the end of the 1990s, the use of solar energy was primarily limited to the construction or renovation of single-family homes, as there were no technically mature offerings for multi-story residential buildings. This has changed. Housing companies rely on solar thermal energy for both new construction and renovation of existing buildings, and many an individual owner of an old multi-family building also makes his tenants happy with this environmentally friendly technology. In view of the constantly rising costs of fossil fuels, it is probably only a matter of time before solar technology catches up with conventional systems in economic terms as well. In environmental and financial terms, the tenants are then on the sunny side of the ledge.

A house in burgdorf in the swiss emmental shows what is possible: last september, the first apartment building was inaugurated there that is supplied with 100 percent solar energy. 276 square meters of collector surface on the roof convert solar radiation into hot water, which a 205.A tank with a capacity of 1,000 liters stores hot water and supplies the occupants of eight rental apartments between 82 and 123 square meters with hot water for domestic use as well as heat for heating. A vertical system of heat exchangers in the storage tank and the design of the heating system as low-temperature underfloor heating ensure that any difference between collector and storage tank temperature can be used. Wooden insulating windows with a heat transfer coefficient (U-value) of 0.5, 22-centimeter-thick exterior insulation on the otherwise conventional limestone construction, and a ventilation system with heat recovery also guarantee that the heat requirement is reduced. Already at the beginning of december it became clear that the system was designed comfortably and that there would still be enough heat available in the storage tank for the rest of the winter. The message the federal project conveys: completely solar heat generation is also possible in apartment buildings.

High growth rates in the use of solar energy for hot water production are also reported from german apartment buildings. Lutz freitag, president of the federal association of german housing and real estate companies (gdw), calculated that the installed collector area at his member companies increased by 55 percent between 2002 and 2005 to 36.000 square meters has risen. In the meantime, the figure is 47.000 square meters. In addition to the availability of appropriate technology for multi-family houses, investment incentives from the federal and state governments as well as favorable loans from the kreditanstalt fur wiederaufbau (kfw) have also provided a boost.

Solar thermal supplements conventional boilers

Since the solar yield in central european latitudes is limited in winter, solar thermal systems are usually only operated in addition to a boiler supplied with conventional fuels – in contrast to the swiss example, which represents a special case as a pilot and model project. Support is provided mainly for water heating, since this does not require the usually high temperatures of the heating water circuit. Solar heat can also be used for heating, for example, to preheat cold water or when the weather and time of year permit lower flow temperatures.

As a rule, the dimensioning of solar thermal systems is based on lower hot water consumption in summer. According to recent studies, this is significantly less than 50 liters per person per day, occasionally even less than 15 liters. According to the experience of the gdw companies, a collector area of one square meter per residential unit is completely sufficient to use solar energy. Larger systems would lead to downtimes in summer and worsen the cost-efficiency ratio.

The state-owned berlin housing company degewo also relied on a solar energy system of this size for an upcoming building renovation in 2001. A building complex in the brunnenviertel residential area, which includes 137 apartments as well as a church, had 165 square meters of solar panels installed as part of an energy renovation project. Together with a necessary boiler replacement and roof repair, this measure yielded 38 percent primary energy savings – in relation to the 137 apartments, a reduction in consumption of 57.000 liters of heating oil per year.

What began in the brunnenviertel has since been continued by degewo in its other housing stock as well. "We already have 19 large-scale solar thermal systems in operation today," says degewo board member frank bielka. Seven more large-scale solar thermal plants would be built this year, making the brunnenviertel, with its 14 plants, the solar center of berlin. The total collector area of 930 square meters will then supply a total of 851 residential units with energy.

According to bielka, the aim of solar refurbishment in conjunction with maintenance measures must be to keep the warm rent stable despite the modernization charge. In view of rising fuel costs, solar systems pay off in the long term in almost every case. Although the cold rent rose by 14 to 20 cents per square meter due to the allocation of modernization costs, the. At the same time, however, the tenants save 15 to 30 cents per square meter in operating costs as a result of the overall refurbishment measures. Warmmietenneutralitat would be given as a rule already starting from the first year. According to bielka, the balance sheet for the rest of degewo's solar refurbished stock looks similar: "we generally reduce the warm operating costs by around 20 to 35 percent and also the carbon dioxide emissions by around 60 percent."

When renovating, it is important to involve a real solar expert, says volker ries, project manager at degewo. He would have to be familiar with the latest technology, have the necessary know-how for planning and implementation, as well as contacts with the solar industry and credit companies. "Without these prerequisites, you end up without nerves and without an economically working solar system."The company has opted for the technical solution of the "solar energy centre" (SEZ). The SEZ combines hot water production and heating support in a central energy system, so you don't need a solar system and another system powered by natural gas or heating oil. Priority is given to the use of available solar energy. Solar surpluses are stored in the connected buffer storage tanks.

The SEZ is standardized and therefore easy to install and handle. For ingrid vogler, this also makes it an efficient and practical solution for multi-storey residential buildings. She is responsible for energy issues at the federal association of german housing and real estate companies (gdw) and the association of berlin-brandenburg housing companies (BBU). Standardization is important, he says, especially in light of the fact that solar thermal systems for large apartment buildings have only been increasingly used since the end of the 1990s, since technically mature options were lacking here for a long time.

Savings higher than the apportionment?

The charlottenburger baugenossenschaft also relies on solar thermal energy and uses the SEZ as its technology. In 2006 and 2007, the company refurbished the meller bogen, a 1970s social housing project in berlin-reinickendorf. To support hot water supply and heating, six thermal solar systems were installed on the roof surfaces, each of which is controlled via an SEZ. Together with better insulation and modern gas heating, carbon dioxide emissions could be reduced by 65 percent and energy consumption by 42 percent. This also reduced energy costs for tenants: although there is not yet a statement of account, all forecasts indicate that the savings will be higher than the modernization levy, says rudolf orlob, technical director of the building cooperative.

In view of rising operating costs and as a contribution to reducing carbon dioxide emissions, the markische scholle housing cooperative also decided to equip its housing stock in berlin-reinickendorf with solar thermal systems. At the same time, the building services in the apartments and basements were comprehensively renewed and the electric instantaneous water heaters were removed. "These were the conditions for a sensible use of solar energy," says board member jochen icken. Within two years, around 570 square meters of solar collectors were installed on 14 buildings. They supply 498 apartments.

The buildings in the second phase of construction on budnerring and raschdorffstrasse were equipped with an SEZ, and the boiler systems were renewed. "Not only the return from solar energy, but also more efficient control of the heating system with SEZ's control technology has reduced overall energy consumption for heating and hot water by 30 percent for these seven buildings," icken said. Tenants benefit doubly from this: "since our cooperative members were already exposed to considerable burdens from noise, dirt and restrictions as a result of the strand renovation, we have waived a rent increase."

Thanks to central water heating, the tenants also save a lot on electricity costs. The switch to solar energy is well received by the tenants of markische scholle. The only regrettable thing, says icken, is that gasag has meanwhile raised gas prices to such an extent that the actual positive effect is not felt clearly enough by its tenants in the utility bill.

The "stadt und land wohnbauten-gesellschaft", one of the big ones among the municipal housing companies, is also switching more and more to the sun. It currently operates eleven solar systems with a total collector area of 1500 square meters for hot water production.

Solar thermal in the old building

As in the big, so in the small: christian kortenkamp, the owner of an old grunderzeit building in berlin's kreuzberg district, also opted for solar thermal technology for an upcoming renovation project in 2004 (mietermagazin 10/07, page 20: "… Und es rechnet sich doch"). The building with ten apartments in oppelner strasse, which was last renovated in the early 1980s, has been thermally insulated and equipped with new heating technology. The old 93-kilowatt boiler was replaced by a modern 35-kilowatt gas condensing boiler. Kortenkamp installed 32 square meters of solar collectors on the roof, which provide hot water for the shower and kitchen and reduce the fuel consumption of the boiler. He reached a voluntary agreement with his tenants that amounts to an average warm rent increase of 30 euros for all the energy-saving measures implemented. Because solar thermal water heating eliminated the need for the power-hungry instantaneous water heaters used in the past, tenants are saving on electricity costs as well as heating costs – the bottom line is that expenses have remained constant.

That one can convert also a monument-protected old building energetically, the housing enterprise gewobag at a building complex in the kreuzberger fidicinstrasse, corner kloedenstrasse showed. In the course of a comprehensive renovation, the water heating was supplemented by a solar system with 40 square meters of collector area. Together with a gas condensing boiler, they generate more than 30 percent of the annual demand for hot water. The thermal energy generated is temporarily stored in water storage tanks and withdrawn from them as needed. The entire renovation reduced carbon dioxide emissions by more than 33 percent. Since the tenants previously had stove heating, a direct comparison of warm operating costs before and after the renovation is not possible, according to a gewobag spokesman. But one can assume, he says, that they have dropped by about 50 percent. At 90 cents per square meter per month, the cost allocation to tenants is somewhat lower than the usual eleven percent, since the building was renovated with grants and loans from the state as part of the "social urban renewal" program.

Huge potential available

In berlin there are around 140.000 apartment buildings and large rental apartment buildings. "But solar thermal systems with a collector area of more than 20 square meters are installed on only about 450 of them," knows kerstin kallmann, division manager at the berlin energy agency. So there is still a lot to do. The majority of tenants would like a solar thermal system on their roof, according to a survey by the german energy consumers association. "Unfortunately, tenants usually don't have the option of deciding which heating system to use in their home," laments johannes lackmann, president of the german renewable energy federation (BEE).

Renewable energy without alternative

From the point of view of the berlin tenants' association (BMV), there is no alternative to the expansion of renewable energies for hot water and heating of apartments. "In the long term, tenants will also benefit economically from the installation of solar thermal systems as energy prices rise," says reiner wild, deputy chief executive of BMV. The berlin tenant association supports however above all those measures, which are warm rent neutral for the tenant.

"What counts for us is what the tenant has to pay in the end," also emphasizes norbert eisenschmid, legal counsel of the german tenants' association (DMB). "If a modern and economically working solar system is installed and the boiler system is replaced at the same time, this will lead to real energy savings for the consumer as well." Then a solar plant is a meaningful investment.

However, eisenschmid demands that the financial support for solar energy must also reach the tenant. So far, this has not always been the case, he says, because landlords are not obliged to make use of them. "Here, instruments must be created that ensure economic efficiency for the tenant even if the landlord does not take advantage of a subsidy."

Udo hildenstab / kristina simons

Solar thermal energy is subsidized

Landlords who install or expand a solar thermal system can apply for subsidies from the federal environment ministry's market incentive program. 105 euros per square meter is available, for example, for solar systems for combined water heating and heating support up to 40 square meters of collector area, for particularly innovative solar systems even 210 euros per square meter.

For larger solar thermal systems over 40 square meters of collector area, there are low-interest loans and up to 30 percent of the eligible net investment costs as a repayment subsidy. New this year are additional bonus programs, for example, for the replacement of an old boiler.

Under the kfw development bank's CO2 building refurbishment program, funds are available for comprehensive refurbishment, and under the "modernize housing" program for individual measures.

With their campaign "natural gas + solar XXL", gasag and the berlin energy agency support the installation of large thermal solar systems when combined with a natural gas-fired boiler. There is a maximum of 5000 euros per solar system.

Solar thermal energy and rent increases

Section 559 of the german civil code (BGB) regulates that the landlord can "increase the annual rent by 11 percent of the costs incurred for the apartment" after a modernization or an energy saving project. However, subsidies received for the investment measure must be deducted from these costs.

To date, there is no known case law that the installation of solar thermal systems constitutes an energy saving that must be tolerated. Until that happens, even a potential rent increase remains controversial. It is unclear whether the saving of primary energy alone, as is the case with the installation of a solar thermal system, already constitutes an energy saving in the sense of the law. Assuming this is the case, then the tenant is subject to an obligation to tolerate under section 554(2) et seq. Of the BGB. After that, measures to save energy and water can not be prevented. The tenant can at most make use of his special right of termination. On the other hand, such modernization requires the landlord to announce the intended measures three months before they begin, as well as information about the duration of the work and the associated rent increase. Since the expenditure for the modernization is compensated with the eleven percent apportionment and fuel costs with a solar thermal plant do not result, such also cannot be made valid – approximately in the form of a "local-usual heat price" – by the landlord as operating cost. However, costs for operation, maintenance and servicing of the solar thermal system are deductible as ancillary costs. With regard to the consumption-based billing of hot water costs, the landlord will make use of the existing meters, because the solar-heated domestic water supply is usually coupled with the conventional system of hot water preparation.

Tenant magazine 7+8/08

Read also
on this topic:

Solar thermal plant on the roof of a multi-family house

Solar thermal system on the roof of an apartment building
all photos: christian muhrbeck

Solar thermal system on the roof of a Degewo building

On degewo's roofs, 19 solar thermal systems are now performing their environmentally friendly service

70s social building of the Charlottenburger Baugenossenschaft with solar collectors

The charlottenburg building cooperative equipped a 70s social housing building with solar collectors

Detail (pressure regulator) of a solar thermal system

Detail (pressure regulator) of a solar thermal system

Solar thermal system on the roof of a house owned by the housing cooperative

The "markische scholle" housing cooperative was prompted by galloping operating costs to carry out a comprehensive energy refurbishment of its housing stock

Detail (digital display) of a solar thermal system

Detail (digital display) of a solar thermal system

Solar thermal system on the roof of a house of the municipal housing company

1500 square meters of solar collector area is maintained by the municipal housing company stadt und land

Detail (solar collector) of a solar thermal system

Detail (solar collector) of a solar thermal plant

Solar refurbishment in Oppelner Strasse, Kreuzberg

Solar refurbishment also pays off for individual old buildings, as demonstrated by an owner in oppelner strasse in kreuzberg

Detail (pressure boiler and storage tank) of a solar thermal system

Detail (pressure boiler and storage tank) of a solar thermal system