Page 488 - A Comprehensive Guide to Solar Energy Systems

Chapter 25 • Optimal Renewable Energy Systems 501

energy storage the electricity need not be produced at the moment the cooling or heating
is required, relieving the need to produce electricity under the most expensive conditions.
similarly, the simplified example includes only large electricity-generating biomass
plants in which at least two-thirds of the potential energy in biomass is lost as waste heat.
Cogenerating heat and electricity in smaller, more widely distributed biomass plants
would allow use of this waste heat and relieve the need to produce electricity for heat
pumps. This would likely reduce the cost of biomass energy, and lower-cost biomass could
enter the optimum portfolio, reducing its cost.
Carbon neutrality in the transportation sector suggests a large increase in the number
of electric cars [32]. While these represent a new electrical load, every electric car includes
a large battery pack, providing opportunities for electricity storage. At a minimum, there
could be some flexibility in charging times, assuming charging stations are provided at
both homes and businesses (or wherever cars may be parked). Cars should be charged
when energy is most available—currently during the night, though this might not be the
case with variable ambient energy. And possibilities go beyond shifting charging times.
Many cars will have battery capacities greater than required for daily use. For example,
a car with a 300 km range might only be driven 50 km on a normal day. With appropriate
incentives, an owner might agree to relinquish part of the battery capacity and its associ-
ated range, at least on some days. A utility could then make use of this relinquished capac-
ity to bolster supply, a capacity that could be substantial given the potential number of
electric cars.
As described in section 25.2, the equimarginal principle implies that the marginal cost
of energy conservation should equal the marginal cost of energy production. From the
example in section 25.4 it can be seen that the value of energy conservation on a critical
day could be very high indeed. Much conservation could be obtained for less than the cost
of energy production [33], and appropriate conservation investments could greatly reduce
total costs.
The Vermont example in section 25.4 also treats daily electricity demand as immuta-
ble, which is another oversimplification. substantial possibilities for load shifting exist, if
appropriate incentives are provided. For example at the author’s home, the electric utility
offers off-peak energy priced at approximately half the rate of peak hours (given the differ-
ent marginal costs of obtaining energy during these times). Peak hours are 6:00 until 23:00 h
Mondays through Fridays, or approximately half of the hours in a week. since subscribing
to this rate, the author’s household has shifted the great majority of its electricity use to off-
peak hours, with about 80% of total usage now being off peak. The electric water heater is
on a timer that heats the tank at night. The dishwasher is only run after 23:00 h and on the
weekend. The clothes washer and dryer have normally been used only on weekends in any
case. There are two electric cars that charge at night. A heat pump which provides back-up
heat (the primary source being wood) runs more during the night, when it is colder outside
and there is no solar gain (and with thermal storage, the heat pump could run entirely at
night). All related controls have default off-peak settings, but can be overridden with the