This June we paid $245.50 for the 1,298 kWh of electricity used in our Connecticut home. Thats a rate of 18.9¢/kWh. The electricity in Connecticut comes from a mix of nuclear and fossil-fueled power plants. The 18.9¢/kWh rate covered only the bill from the power company. It did not include the taxes I pay to finance the military, which is protecting our fossil energy supplies, and it did not cover the collective price we all pay for global warming.
In comparison, thermal solar power plants generate electricity at 15¢/kWh; photovoltaic plants at 20¢/kWh. Solar energy is free and inexhaustible. Furthermore, the decision to cover 10 million American homes with solar roofs would trigger the biggest economic expansion of the decade and generate energy savings exceeding the imports from the Persian Gulf.
If in the suitable areas of the country, solar roofs generated free energy that was stored on the grid, the energy consumption of private homes could be reduced to zero in most areas.
According to a 2007 study of the Mc.Kinsey Global Institute, the residential energy consumption in the U.S. is 21.3 quads (Q = 1015 BTUs), and it can be reduced to half by installing double windows, improving sealing, thermal insulation and the efficiency of lighting and appliances. Operating appliances consume 8.5Q; heating/cooling, 7.5Q; hot water, 3.2Q; and lighting, 2.1Q. The study concludes that efficiency improvements alone can reduce the energy consumption of American homes to 13.5Q by 2020.
In some areas, the renewable energy supplies are insufficient and have to be complemented by conventional energy supplies, while in other areas zero-energy homes can be built. In order to supplement the solar energy, these energy self-sufficient homes can also use geothermal and/or wind energy. According to my calculations, in most locations, the zero-energy home can also provide the energy needed for plug-in hybrid or electric cars.
Naturally, today we are a long way from having 10 million zero-energy homes, but the process of conversion has already started. Many electric power companies accept the excess electricity generated by renewable energy sources into their grid for a credit (the electric meter is running backwards). This is beneficial to both parties, because the times of excess solar generation coincides (because of the air conditioning load) with the periods of peak demand. This way, when more energy is being generated than needed, the homes owner does not need to be concerned about energy storage, while the power company can reduce the otherwise-very-expensive peak power requirement of the system.
The zero-energy household (Figure 1) can later use this accumulated credit to receive free electricity when at night, or during the winter, solar or wind energy is not available. When specifying the size of the solar collectors, it is advisable to double the maximum expected requirement, so that sufficient excess electricity will be generated in the summer to zero out the electricity bill for the year.
Most of the private homes of Europe or the U.S. could take advantage of inexhaustible, clean and free forms of energy solar, wind and/or geothermal. (Go to www.controlglobal.com/lessonslearned.html for a map showing the free energy potential of the U.S.) Their use could eliminate completely the emission of greenhouse gases, which today amounts to 10 metric tons (22,000 pounds) a year for the average American household. This is five times the global average.
Naturally, the initial investment needed to convert theses energy forms into electricity can be high. Many governments subsidize the use of renewable energy systems. The payback periods of alternative energy installations ranges from five to 20 years, solar hot water systems being the least expensive.
The payback period for installing a photovoltaic electricity generating system, based on a home with a monthly electricity bill of $100, an installed system cost of $50,000 and a rebate plus tax credit of $20,000 in California, is about 15 years. If the monthly electricity bill is $250, the payback period drops to about eight years, and factoring in the increase in the homess value, it drops to about four years. These payback periods were calculated on the basis that the electricity cost in the area is 12¢/kWh and will not rise. (This cost in my area is 18.9¢/kWh and rising.)
The generating capacity of the average solar collector system for private homes ranges from 5 kWp to 10 kWp. The yearly energy bill of an average household in the north central U.S.is about $4,500. (Links to energy usages sites are at http://www.controlglobal.com/lessonslearned.html.)
The technology for the zero energy home is readily available now.
Today, there are only some 50,000 solar homes in the U.S., but their number is increasing at a rate of ten-fold per decade. As experience is gained and production volume rises, the cost of collectors is dropping by about 5% per year. In the U.S., some 90% of the existing solar collector installations generate only hot water, while about 10% also generate electricity.
The renewable home energy installations can also use household wind turbines, which are usually small (2kWh/day 10 kWh/day at 10 mph wind speed). A 10 kWh/day unit costs about $8,000 if used to charge batteries, while one tied to the grid costs about $12,000, including inverter.
There are also a few solar and geothermal combination homes, such as the Strizki home in New Jersey. The United States Merchant Marine Academy has one of the first solar-hydrogen generating installations.
Another likely feature of the private homes of the future will be continuous optimization and wireless remote access. Energy optimization will be provided by computer software packages that continuously watch the cost-effectiveness of operation. For example, if a home has both an old oil furnace and a solar-powered hot water, electricity and geothermal heat pump, the optimizing software will continuously maximize the use of the least expensive form of energy.
For example, in the winter, the storage of solar hot water will be exhausted before starting the geothermal heat pump, and the furnace will only be used as needed to supplement these sources. Similarly, in the summer, this control software will cool the home by operating the geothermal heat pump in reverse and by using solar electricity to drive it. During all seasons, the system will watch the cost of grid electricity continuously and make the decision on selling or buying energy from the grid on that basis, taking advantage of peak energy costs.
This optimizing software can also be integrated with miniature cameras, smoke and security alarms that will automatically contact the owner in case of an emergency. In addition, it can also allow the owner to view or change the conditions in the home using a wireless mobile phone to adjust thermostat settings and stop and start equipment.
Go to www.controlglobal.com/lessonslearned.html for more resources.
|About the Author|
Béla Lipták, PE, process control consultant, is editor of the Instrument Engineers' Handbook and a gormer chief instrument engineer at C & R (formerly John Brown) and former adjunct professor at Yale University. He can be reached at firstname.lastname@example.org.