In the US during 1973-1984, the sudden increase in energy costs triggered a greater attention to energy management in buildings, from residential to commercial. As a result of this consciousness, buildings greatly improved in energy efficiency in aspects such as wall insulation, improved window design, improved heating and cooling systems. There were typically 40% improvements in the energy efficiency of commercial buildings [Solar Energy Research Institute, 1988, pg. 9] to an annual 126kWh/m2 (40 kBtu/ft2) and a similar improvement in new or rebuilt residential buildings to an annual 25 kWh/m2 (8 kBtu/ft2).
In addition to these dramatic energy efficiency improvements in building construction technology, there have recently been improvements in energy management, with set back thermostats, improved appliance efficiency, occupancy sensors, and improving occupant attention to saving energy as a fundamental philosophy. As has been recently demonstrated in California during severe energy shortages of electric power, residential owners were able to save consistently 25% as just being more diligent in energy usage in their daily activities. Although no hard data was found, it is believed that energy reductions for residential buildings have recently dropped to an annual 15 kWh/m2 (4.8 kBtu/ft2). This compares most favorably to the energy-efficient European house of 15-30 kWh/m2 per year (EUREC, 2002).
On a floor area basis, the Berkeley house with swimming pool operation uses a total annual energy of only 8.6 kWh/m2 per year (0.8kWh/ft2 per year). Since there was absolutely negligible use of natural gas, this figure represents the total
Figure 10.1 Long-term trend in residential energy efficiency.
energy use. Above all, further reductions in residential energy use can be made when solar energy or wind are used as local energy generation sources. As seen in Chapter 6, the Berkeley house had about 25% of its annual electrical energy use supplied by solar PV.
In the US 25-40% improvements in residential efficiency have been noticed immediately following 2000 as can be seen in Figure 10.1. In addition, a continuing trend is foreseen where the US residential housing stock will continue to move toward high energy efficiency and will join the already high efficiency EU housing. This trend is projected in Figure 10.1 for total energy use over many decades toward 2050 when the residential energy efficiency is expected to reach about 9kWh/m2 per year, with around 25-30% of residential energy coming from solar passive design elements as well as active solar thermal and PV. So a bright solar future is ahead worldwide.