In the electrically heated residence, as show in the diagram on the left, the heat rejected by the power generator would be greatly in excess of any heat that could be useful in the home during all seasons of the year. This mismatch is large enough to exclude electrically heated homes as candidates for CHP systems.

In the gas heated home the match between available rejected heat and useful demand comes closer, but is still unfavorable. In the winter months the power generator could only supply a small amount of the required heat and in summer, due to the air conditioning electrical demand, it would produce more heat than is needed.

This non-coincident load situation can be ameliorated by replacing the electrically powered vapor cycle refrigeration system with an absorption chiller, which effectively converts the electrical draw for air conditioning into heat demand. While the power generator has far less rejected heat capacity than required, there is almost no heat wasted.

In each of the three cases the total energy demand adds up to about the same amount, but the efficiency with which it is delivered varies dramatically

In the electrically heated home with electrically powered air conditioning the net loss of heat energy would add up to 103.7 million Btu (1,037 Therms - $725) for the year [46% overall efficiency]. If utility electricity from a grid with a system efficiency of 29% were imported, the equivalent loss would be 146 million Btu (1,460 Therms ~ $1,000). In the gas heated home with electrical air conditioning, the loss would be 32 million Btu (320 Therms - $200) [72% overall efficiency]. In the gas-heated home with an absorption chiller there would be almost no surplus heat generated [~100% overall efficiency].

The graph on the left provides the same statistics for a multi family residence in a south central setting as the second graph in the previous single family discussion.

Due to the sharing of common walls, multi-family dwellings tend to be easier to heat, but the air conditioning loads per residence remain nearly the same.

Hence, multi family residences will  yield to the system solutions for single family homes.

Load Fluctuations

Daily totals do not reflect load swings that are crucial factors in the design of a CHP system.

The plot on the left displays the swings in demand that occurred each day during the first half of 2002 in the south central region. The high load figures identify mostly with the winter season. Following the peak demand line, it will be seen how load changes rise steeply in the morning, drop off during the day-time and rise again in the evening. In this instance the data pertain to an electrically heated home, but had the time-of-day demand for a gas heated home been measured, the profile would be the same. In more temperate seasons, the amplitudes are reduced as may be expected and there are months in which the day-time energy demand is a steady 1 kilowatt draw or less.

On top of that, residential electrical demand is subject to sudden surges as appliances are turned on and off during food preparation, in the course of a family getting ready for the day, or to venture forth in the evening. It has been said that the bane of the home CHP designer is the contemplation of several family members turning on their hair driers at about the same time, each representing an instantaneous load of over a kilowatt! Not only does it influence the peak output capability of the generator system, but also the heat absorption side of the equation, since there may be little or no coincident heating requirements when the peak occurs.

System Integration

The energy use in a conventional American home is accounted for by a number of independent utilities and accessories each powered separately by electricity or gas. Even the built-in utilities operate independently of one another. To minimize the use of energy for optimum results clearly requires an integrated approach to the design of a domestic CHP system.

The schematic depicts the type of system envisaged. It is obvious that such a system should be designed into a home rather than be retrofitted since there has to be a large measure of common use between sub-systems.

For example, instead of each heating device having its own burner or electric heating element, all devices could share a common source of heat, namely the heat rejected by the power generator. The greater this type of integration, the bigger the energy savings will be. For instance as in the diagram, the clothes drier and the cooking range could be part of the system to take best advantage of the heat energy left over from power generation.

Power Generator Design Considerations

Taking a systems approach, the power generator is the essential core of the system. It must be able to operate efficiently during periods of minimal heat demand and under low partial electrical load circumstances, yet have the capacity to absorb sudden power demands. On the opposite side of the spectrum, it must be able to respond to high heat loads even when electrical demands are minimal.

It has to be extremely reliable, require virtually no maintenance, have long endurance characteristics, be virtually noiseless, have ultra-low emissions and most importantly, it has to be economically attractive.

Design Parameters (tentative)

After reviewing single family residential load profiles for different regions of the country and various utility configurations, excluding electrically heated homes, the following tentative design parameters were formulated:

Electrical Capacity        5 kW at peak efficiency;

                                    Up to 10 kW with some erosion of efficiency;

                                    Down to 500 watts within 20% of peak efficiency

Heating Capacity           20 kBtu/Hr at peak generator efficiency

                                    68 kBtu/Hr Maximum

The best long endurance, small power generator with sufficient part load tolerance to meet these requirements comes in the form of an externally fired, multi stage, closed-loop, Brayton cycle driving an integral high speed wild frequency generator.

A two-stage design is depicted in the schematic. It consists of a closed-loop, recuperated system utilizing external combustion. Power output is varied by controlling the air density in the loop with a fill and drain controller.

This scheme maintains high thermal efficiencies over a very wide partial load range, including small fractional loads.

It also has the advantage of increasing the combustor output to match the thermal load in one of two ways.

§         The first is to load the generator with resistors placed within the space heating system which increases the heat flow from the power plant in addition to the electric heating.

§         The second is to partially bypass the recuperator in the closed loop duct some combustion air to a separate heat exchanger in the heating system.

Another advantage of external combustion is that it allows the use of low-intensity, surface combustion techniques which result in ultra-low nitrous oxide emissions. Finally, it is easily adapted to use a wide range of fuels.

The system is mechanically simpler and thermally more efficient than equivalent internal combustion power plants such as reciprocating gas engines.

Because it is a closed loop the noise is captured within the loop which results in a very quite exterior noise field.

The high rotational speed of the two turbo stages ensures that very low vibration levels are achieved.

In quantity production both stages will be supported on air bearings to achieve very long service life and to remove the need for lubricants and lubrication servicing.

There are important advantages in making the rotors of the turbo stages out of silicone nitride and the stators, combustion heat exchanger and combustors out of ceramics. The specific weight of silicone nitride is about the same as aluminum which lowers the inertia of the rotors considerably and since ceramics have very low coefficients of thermal expansion, it becomes possible to maintain much closer clearances between the rotor and stator elements, thereby increasing the efficiency of the turbo stages. By going to ceramics, higher turbine inlet temperatures can be sustained which improves cycle efficiencies still further. Finally, the repetitive production of these parts out of ceramics costs less than metal equivalents. The rotor elements are of a size smaller than internal combustion engine turbochargers which have for years been manufactured successfully in large quantities out of silicone nitride.

Reliability Considerations

The same fundamental reasons which allowed gas turbines to bring new dimensions to aircraft availability apply to dynamic machines in utility use. Radial piston aircraft engines had overhaul lives of 1,000 to 1,600 hours. In the piston engine era in-flight failures were so common that at least four engines were required for trans-ocean passenger flights. In contrast, jet propulsion engines routinely average 10,000 hours between overhauls and current civil transport jets require only two engines for the same flights.

3,000 rpm internal combustion engines driving stationary generators offer overhaul lives of 10,000 hours, with the best slow-speed diesel generator engines going as high as 16,000 hours. Gas turbine generators, weighing much less, offer significantly longer lives between overhauls. Stationary gas turbine installations commonly operate continuously for 75,000 hours without overhaul. In rugged applications on off-road heavy equipment turbo chargers outlast their diesel engine hosts by several overhaul cycles.

Micro turbines, such as the Capstone unit log 30,000 hours and more without unscheduled service interruptions.

Community Electrical Backup Service Alternatives

While the system design is based on stand-alone operation independent of any electric utility, and while power losses are expected to occur far less frequently than utility outages commonly happen, the acceptance of residential distributed energy systems would gain significantly from having reliable backup provisions. Public or investor-owned utilities have reason to charge interconnection fees for backup support of co-generators, on the theory that they are then compelled to keep peak load capacity in reserve should backup service be required. The experience has been that the interconnection process is used as a technical stumbling block to discourage the use of CHP systems, and that public utility commissions seem motivated often to allow excessive backup fees to be levied. Clearly, utilities with large capital investments to protect have no reason to encourage customer flight from their systems.

The best solution may be to create so-called “mini grids” in which groups of homes or even whole “green” communities are linked together, for backup purposes only. This would be particularly practicable in multi-family housing developments.