The word heat pump is a collective term for a wide range of products utilising the same working principle. There are however many different types of heat pumps, all of which most suitable for different applications. Heat pumps are in general divided into different types depending on which heat source and heat sink they are designed for. All types have their own pros and cons as well as environmental impact. The following section will present the general principle of heat pumps that is common for all types considered in this study. There after an overview of available heat sources, aspects related to system design and viable efficiency will be given.