To gather information about the heat and power plants the following methods have been used:

• internet search at the website of the plant operator

• contact via fax and e-mail (with a special questionnaire)

• telephone calls

• pictures of the plants via Google Earth

• in some cases also on site visits

For a general overview of the existing heat and power plants, an internet search was carried out. This proved to be very effective because almost all heat and power plant operators have web sites[2]. Some technical parameters like installed power and heat capacity or produced energy can also be found in the internet. However, no information about the district heating net hydraulics (open, closed or mix systems), flow rate of cold water to be refilled, temperature level or operation mode can be obtained in this way. Thus, internet search alone does not provide sufficient information for an evaluation.

In a second step, heat and power plant operators were contacted via fax or mail with a special questionnaire about technical parameters. Unfortunately, but not completely surprising, no email or fax has been answered. With a telephone survey it was possible to obtain further technical information (like net hydraulic and summer operation) about the heat and power plant and district heating net. In general, it was unlikely to get information about the cold water flow rate, operation mode and energy generation or fuel costs.

Additionally, Google Earth was used to estimate the available area for installation of uncovered collectors. However, in all cases an on-site visit was found to be inevitable in order to gather all data required for a full evaluation. Figure 2 shows a comparison of the data collection methods used with regard to the data obtained.