Stockholm, Sweden
The Stockholm district heating system is large with over 12 TWh of heat
demand annually. Since the 1970s the system has been operated with both
combined heat and power (CHP) and heat pumps (HP). About 660 MW of
heat pumps and 300 MW of electric boilers are currently operational in this
system.
The operation of the district heating system is optimized taking into account
the heat demand, electricity and fuel prices and congestions in the district
heating network. In figure 1 a normal yearly duration curve for the
Stockholm district heating system can be seen. CHP with different kinds of
waste as fuel (blue area) is the bas load production together with CHP with
wood chips as fuel (dark green area). The coal fired CHP (black area in
middle) is today (2022) replaced with bio CHP. Normally HP is the next
generation source (white area). Above HP comes CHP with pellets as fuel and
solid bio heat only boilers (HOB) and turbine bypass (light green area). On
top is bio oil and then fossil oil HOB (black area on top). On daily basis
depending on electricity prices and bio fuel prices HP can replace bio fuel
CHP or be replaced by pellets bio fuel CHP. When the electricity prices are
extremely high HP can be replaced by solid HOB and turbine bypass. And
also the other extreme with very low or negative electricity prices can result
that HP replace wood chip CHP.