Definition: regenerative sources of energy are accordingly characterized as 'sources of energy which renew themselves within human time scales'.¹

There are three central sources of regenerative energy on earth²:

1. Solar energy (solar radiation)
2. Geothermal heat flow
3. Gravitation of the moon (tides)

isoplus products are being used more and more where heat (or cooling, i.e. district cooling) has to be transported relatively far distances. This transportation can involve simply a few metres or many kilometres. The option of spatially separating production and supply of heat (or cooling) makes the use of many regenerative sources of energy possible. In addition, the use of energy by means of this separation can be arranged in a more environmentally friendly and convenient way for the consumer. The reasons for this can be for example:

• Use of large geothermal resources
• Integration of complex technology for treatment of waste gas, which is often only economically viable in large systems.
• Use of fuels whose use is combined with increased operational demands (i.e. certain forms of biomass). For consumers, district heating constantly functions fully automated.
• Integration of large heat storage facilities in the supply of heat. With heat storage facilities, fluctuations in the availability of heat during the course of the day can be regulated (i.e. with large solar power systems). Also, heat storage facilities can be used in order to temporarily convert excess production of electricity (i.e. from wind power or photovoltaic) into heat and use it efficiently for heating purposes. The storage of heat can be less costly in individual cases than the storage of electricity, for example in batteries or pumped storage power stations.

The supply of energy from regenerative sources is frequently more structured in a very decentralized form. Consequently, this gives rise to particular challenges but also opportunities in the energy sector.