One of the most serious problems with our current energy system is that it does not consider the idea of exergy - the amount of useful work which can be produced from a given fuel. In our buildings we use fuels to create heat for comfort purposes, diluting all of the potential to produce useful work down to essentially useless 72 degree space heat. In contrast, we burn fuel at power plants, taking the useful work (electricity) and throwing away the dilute heat. We heat our dwellings the same way we make birdhouses: by chopping prime lumber into little itty-bitty planks and wasting all of the potential to build bigger things. We make electricity the same way we build houses to live in: use the "useful" parts of the lumber and throw away all of the scraps and excess.

What if we could build a real big house, and then use the leftover scrap wood to build birdhouses? We can - it's called cogeneration and it is certainly not a new/expensive/radical concept, it is just one which we have been reluctant to implement for various reasons - cheap fossil fuels, climate (in the case of the South), dispersed population, pressure from electric utilities, etc.

In the United States, Canada, and many other countries, the penetration of district heating never really progressed as fossil fuels were always cheap and subsidized and the populations are so dispersed in suburbs as well as rural areas. It is for the most part completely impractical to run district heating to these areas. Electricity, however, can and is easily delivered to such areas.

Barring some radical change in the ways people live (e.g. moving from mega-cities and suburbs into smaller walkable cities as is promoted by James Howard Kunstler and others), it will be necessary to modify the energy infrastructure to accommodate the population distribution while maintaining a high level of exergy utilization.

...So if we can use district heating and/or small-scale building-based cogeneration in the cities, we should be concentrating thermal electrical generating plants in urban areas rather than in faraway middle-of-nowhere settings as has been the practice since the 1950s. This allows as much electrical energy to produced from the fuel as possible and then a market for the remaining heat. Natural gas is, obviously, the most desirable fuel and in most situations would be the only option barring biomass or solid waste generated within city limits.

Combined-cycle gas plants actually produce more electricity than heat, and since most urban societies today with district heating consume far less electricity (on a joule-for-joule basis - see Denmark) than heat, an electricity surplus would be available for export to rural areas. Homes and businesses here would be able to utilize electricity in place of natural gas for heating and cooling through the use of heat pumps. By converting as much natural gas energy into electricity as possible, a much better utilization of the energy is realized. Using natural gas for space heating is wasteful. I don't care if a gas furnace/boiler is "98% efficient", it is still essentially a machine that grinds steaks into hamburgers (to use Deffeyes' analogy). That is wasteful.

URBAN ENERGY CONCEPT: GENERATE POWER IN THE CITIES, SEND WHAT'S LEFT OUT TO RURAL AREAS.

Those rural buildings (as well as the urban centers, though it would be less urgent due to the wide availability of excess heat from the power plants) would also greatly benefit from solar-thermal panels, reducing the need to use electricity for dilute heating purposes whether it be through the use of resistance or a heat pump. Surpluses of electricity are unlikely to be an issue, especially if electrified transportation were to come on stream. Renewable electricity from the various hydro, wind, solar thermal, and geothermal installations would have to be introduced into the system through a transmission grid.

Climate Effects

The effectiveness of such a system would depend upon the climate; northern cities would benefit much more than southern cities due to the colder climate creating a larger demand for heat. During summer and in the South, electricity is the dominant form of energy consumption - driving all of those central air conditioning units in Sun Belt suburban homes while their occupants are at work is a serious electrical black hole. Higher ambient temperatures also reduce the efficiency of thermal power generation schemes (Lower delta T = less efficiency).

Aside from common-sense demand-side measures (e.g. white roofs in the South, passive solar buildings, ice storage, turning OFF the air conditioner when building or home is not occupied, reducing the very common OVER-cooling of buildings, etc.), summertime demand could be met with energy from large-scale solar-thermal generation and rooftop PV systems. Those systems fit the demand profile of air conditioning very nicely. Even if the urban power generation schemes continued to supply full power, the gas energy loss as dilute heat would be at most 40% and the city occupants would continue to be supplied with domestic hot water through the district heating network.

Waste heat and power plant

Aside from standard natural gas combined cycle cogeneration plants, a facility including a waste-to-energy system for solid waste treatment could be integrated to utilize the energy present in waste.

• Municipal waste residues collected from the urban population is hauled to a resource recovery facility and sorted. Residual non-recyclable portions are incinerated. It may be better suited to have multiple smaller WTE installations utilizing oscillating kilns rather than large regional plants. The smaller footprint and daily tonnage allows the plant to better fit into an urban environment.
• The heat from the incineration lines is used to produce steam at a temperature of no more than about 725 K (if I do recall correctly these are values typical of modern waste-to-energy plants).
• Natural gas is burned in a gas turbine to produce electrical energy.
• The exhaust heat from this gas turbine is used to boost the temperature of the steam from the incineration lines to higher temperatures and pressures.
• The steam is then used to drive a steam turbine to produce additional electrical energy.
• The turbine exhaust is condensed and the remaining heat dispatched to the district heating network.
• The flue gases from the incineration lines are condensed (burning high-moisture waste along with vapor from the scrubbing process) and used to preheat the incoming district heating water.
  • An interesting technical question regarding flue gas cooling/condensation is the mechanical work required to force cooled flue gases up the stack. If we extract as much heat as possible from the gases, it may appear that we have increased the efficiency of the plant. However, if we then need to use more electricity to induce a draft to get the now less-buoyant gases up the stack, could the efficiency be less? Electricity to drive a fan is much more valuable than a couple hundred degrees of heat in flue gases. Removal of water, however, decreases the mass flow rate up the stack and may reduce the work needed to pump the gases. It would be interesting to see the result on the new WTE plants in Europe built with gas condensation and pencil-thin stacks.
• Wastes may be baled and stored during summer when demand for district heating is low, then burned in winter to provide additional heat.