Within the scope of biomass energy two small district heating networks, each based on a 150 kW pellet boiler, have been installed. A Cemetery Chapel (30 kW) and a hibernation greenhouse (600 kW) have been equipped with a wood pellet boiler.
The municipal service buildings at “Burgweg Depot” and the “School Biology Centre” have also been equipped with a pellet boiler. A photovoltaic system has been mounted on the storage building at “Herrenhausen power station” and produced 70 MWh of electricity in 2011. At a public open air swimming pool solar collectors with an absorber area of 1,800 m² (600 kWpeak) have been installed and are supplying heat of about 500 MWh/a.
At Vahrenwald and Ahlem altogether 305 apartments (originating from 1950 – 1970) have been retrofitted to meet the energy standards of new buildings. Another 35 apartments (built in 1956) have been retrofitted according to the Kronsberg standard with innovative Passive House components to reduce final energy demand for heating to 55 kWh/m²a. The “Brothers Grimm” primary school (5,356 m²) has been retrofitted to reduce heating demand by 50% (Picture 1). A photovoltaic plant of 83 kWpeak has been installed on its roof. The “Cultural Centre Hainholz” (2,157 m²) and secondary school “Luther School Nordstadt” have been modernized by energy efficient measures.
The coal fuelled CHP (combined heat and power) plant, which supplies a large existing district heating network, is now partly based on wooden biomass. 1,400 MWh of heat, now produced by renewable energy sources, cover the energy demand of the act2 eco-buildings.
In 2007 the “Wood Energy Centre” was commissioned. On the site of the Hannover-Stöcken power station a modern log-splitting plant is operated (Picture 2), which directly uses surplus heat from the power station to dry the wood. The local ESCO „Stadtwerke Hannover“ sells wood products for various types of wood boilers or ovens: pellets, chips, logs and wood briquettes.
Solar Thermal Collectors for Open Air Swimming Pool:
Hannover’s largest outdoor swimming pool “Lister Bad” is also one of the most popular in the city; on hot days more than 10,000 people visit the pool. To keep the pool water temperature at 22°C the annual average gas consumption (two boilers with each 650 kW) has been about 1,350 MWh. However as energy costs increased steadily in recent years the city facility management department looked for new solutions to reduce costs, energy consumption and environmental impact.
An absorber area of 1,774 m² (600 kWpeak) was installed in the first quarter of 2007 (Picture 3). 160 m³/h of water are pumped directly from the different pools through the absorber and back into the pools. Since commissioning the solar plant has run without any significant problems. In case of insufficient solar energy the two gas boilers complement the heat generation. Table 1 shows the monitored figures during four years. The overall cost of the installation of the solar collector and some additional constructional work amounted to 209,000 €, of which 140,000 € (approx. 80 €/m²) was for the collector. The annual cost savings for the first two years of operation have been more than 100,000 €. Payback period of the whole installation will be about three years (including subsidies).

Table – Figures from year 2005 until 2008
Small-scale District Heating based on Biomass:
The Burgweg depot has offices, workshops and stores for traffic control equipment (traffic lights, signs etc.) and highway maintenance. The total usable heated area is about 800 m². The heating energy had been provided by five separate oil-fired boilers installed at different buildings. The total installed capacity was around 570 kW.
The separate boilers have been replaced by a small district heating network. The existing boiler room was extended to provide space for a pellet-fired boiler, a thermal storage tank, a backup oil-fired boiler (for peak demand) and a distributor. Neighboring buildings were connected by new district heating pipes and a pellet silo of 10 m³ was built outside in front of the boiler room (Picture 4). The payback period will be around 10 years (subsidies included).