Energy-efficient hot water generation through decentralized heat pump systems
11/2012 to 11/2014
Federal Ministry of Transport, Building and Urban Development (BMVBS)
Since, due to technological advances, heat pumps reach higher and higher levels of efficiency, the project “Energy-efficient hot water generation through decentralized heat pump systems and central regenerative heat supply” investigated possibilities for efficient heating of drinking water.
To ensure hygienic conditions in drinking water pipes, it is especially important to curb Legionella growth. As a means to control the growth of Legionella in drinking water tanks, the DVGW’s worksheet W 551 governs, among other things, a minimum temperature of 60 °C at the outlets of water heaters and central flow water heaters.
Since the performance coefficient of a heat pump is essentially dependent on the difference between the source and the sink temperature, heat pumps are less efficient for the purpose of hot water generation than they could be. One way to reduce the required temperature for domestic hot water systems is to rely on decentralized water heating. For decentralized drinking water heating systems with a downstream volume of less than 3 liters – as in electric heaters – it is sufficient (from a thermal and hygienic point of view), to provide drinking water at lower temperatures. To make use of these favorable conditions, it was investigated whether domestic water heating could be achieved by a decentralized system based on heat pumps. The aim is to keep a central water storage tank at a temperature level of about 30 °C by means of renewable energy (in-house waste water energy, for example). This energy storage feeds a circulatory system with a supply and a return pipe inside the building. First, the cold drinking water is preheated at the decentralized domestic hot water taps by means of a heat exchanger within the circulatory system, and is then heated to a tap temperature of about 45 °C by means of a compact micro pump at the faucet. The circulatory system serves as a heat source for the micro heat pump. Here, the low flow temperatures and the low heat losses in the heating circuits (due to the comparatively low circulation temperatures) lead to a significant potential for energy savings. Thanks to short pipelines between the decentralized water heaters and the taps, there are no additional measures needed to combat Legionella, as there is no warm water storage involved. Installing decentralized micro-pumps within buildings can lead to further CO2 savings in the building sector, providing domestic hot water under ideal hygienic conditions at the same time.