Heating and Cooling Energy - Hydrogen and Fuel Cells: Advances in Transportation and Power - page 238

turn mains carry the cooler, used water at 120°F back to the central facility.
Costs can be lowered with the use of newer types of pipes, insulating
materials and excavation techniques. Plastic piping in long rolls is laid in
plastic insulation and placed in narrow trenches. Using these techniques,
hundreds of feet of pipe can be laid quickly. Metal radiators can also be
replaced by plastic units.
District heating systems are often financed by municipal bonds at
low interest rates, to be repaid over a 30- to 40-year period. This makes the
annual cost per home competitive with or less than that of conventional
heating systems.
A seasonal energy storage system is designed to store heat or cold
during one season, when it is not needed, for use during another season.
These systems have a large energy storage component. They collect es-
sentially free heat or cold when they are plentiful and save them until
required. The only energy consumed is that needed to run the various
parts of the system. Three types of systems exist: annual cycle energy sys-
tems, integrated community energy systems and annual storage solar dis-
trict heating. The first two can provide both heating and cooling while the
third is used for heating only.
The annual cycle energy system (ACES) has two basic components: a
very large insulated storage tank of water and a heating-only heat pump.
The tank contains coils of pipe filled with brine (salt water) warmed by the
water in the tank. The brine circulates through a heat exchanger and trans-
fers its heat to the heat pump refrigerant.
During the heating season, heat is removed from the water tank by
the brine and transferred to the building at a temperature of 100 to 130°F.
The system may also be used to provide domestic hot water. As heat is re-
moved from the tank, the temperature of the water drops below the freez-
ing point and ice begins to form on the brine circulation coils. By the end
of the heating season, ice fills the entire tank. This ice is then used during
the summer to provide chilled water for air conditioning. While the ice re-
mains in the tank, the only power required for cooling is for the operation
of a circulator pump and a fan.
In actual installations these systems have been shown to use about
45 to 50% of the electricity consumed in a similar house with conventional
electric resistance heating. It is more efficient than a conventional air-to-
air heat pump system, since the heat source is maintained at a constant,
known temperature. In moderate cold climates with 6,000 degree-days, an
ACEs uses about 25% less electricity than a conventional heat pump with