|A cement kiln waste heat recovery power plant|
By Robert W. Timmerman PE, CEM, LEED AP
Buildings need both electric power and heat. Electric power runs lights, computers, and air conditioning; heat warms the building in winter, and heats domestic hot water year round. The total energy used by a building is the fuel burned to make electric power plus the fuel burned on site for heat. On a nationwide basis, about 30% of the total energy used by buildings goes for heat. The percentages are higher in the Northeast and the Midwest. To make buildings fully renewable, a renewable supply of heat is needed in addition to a renewable source of power.
The Thermal Smart Grid
The Thermal Smart Grid would supply much of the heat energy needed by buildings from renewables such as solar heat, and by recycling otherwise wasted heat from power and industrial plants,. The various sources of heat would be coordinated by real time controllers combined with a sophisticated rate schedule which would encourage the optimum use of resources to match each building’s particular need.
Availability of these sources would be enhanced by interseasonal thermal storage, storing heat in summer for use in winter. This technology has been demonstrated in Europe, and designs have been prepared for an interseasonal storage project in Massachusetts. The economies of scale with thermal storage give the Thermal Smart Grid a natural advantage over smaller scale systems. Interseasonal thermal storage extends the amount of heat available, and might make possible the storage of winter cold for efficient summer cooling. The same distribution system could distribute cooling in sumer.
Potential Energy Savings
By recycling the waste heat from fossil fired power plants within economic thermal transmission distances of population centers, the Thermal Smart Grid would save about 5% of the total energy used by buildings, and would reduce carbon dioxide emissions by about 75 million tons per year. Use of other heat sources would increase the savings: a combination of solar energy with interseasonal storage could increase the savings to perhaps 12% of the total energy used by buildings, which would be 40% of the fuel burned to heat buildings.
The Parts of the Smart Grid
Just as the Electrical Smart Grid, the Thermal Smart Grid is a system, that includes sources of heat, heat customers, piping to interconnect them, short term and long term thermal storage, and controls to manage the system and interconnections. The grid itself would be a piping network operating a relatively low temperature in order to obtain heat at high efficiency. At the design maximum water temperature of 120F (50C), heat is available at reasonable efficiencies from simple solar collectors and from recycled power plant and industrial waste heat. Use of water at this temperature for heating is an established technology.
Just with electrical grids, an overall control system would manage the Thermal Smart Grid. This would take the form of an executive controller that would match the daily demand for heat with the resources available. Surplus heat would go to storage, and shortages would be made up from storage, or from local heating systems. Overall control would be assisted by an innovative rate schedule, where customers contract for a fixed flow rate of hot water, and are billed for the quantity they use, as well as the monthly bill for the quantity. This gives the customers the flexibility to optimize how their buildings use heat.