Systems Research

Energy Efficient Systems

Distributed Energy Resources

Distributed energy resources (DER) are usually defined as relatively small systems (1 kW to about 60 MW) located on the customer's side of the meter or within the distribution portion of the utility's generation, transmission and distribution (T&D) system. Strategically located DER can help to reduce the customer's demand for power and energy, provide grid support, and deliver power directly onto the grid. DER includes distributed generation (DG), demand-side management (DSM), energy storage (ES) and, potentially, vehicle-to-grid (V2G) systems.

The initial focus of Stanford's efforts will be to focus on the implications of DER implemented on the customer's side of the meter. More specifically, we will investigate the interactions between the grid, an aggressive DER program addressed at residential and commercial buildings (which account for roughly one-half of all U.S. energy), and the near-future possibility of plug-in hybrid, all-electric, and fuel-cell-powered personal vehicles.

Distributed Generation (DG) technologies include combustion turbines, microturbines, fuel cells, photovoltaics, reciprocating engines, wind-turbines and Stirling engines. Larger systems may be located at distribution substations to support the grid, while smaller systems may be located on the customer's side of the meter to reduce demand and/or to increase reliability. Customer on-site generation has several efficiency advantages including reduced power-line losses and for some the possibility of combined heat and power (CHP), which can reduce primary energy demand by 40 to 50 percent compared to separate grid electricity and boilers.

Of the approximately 250 GW of currently installed DER in the U.S. (defined as generation less than 60 MW), over 80 percent are reciprocating engines. Air quality permitting constraints however often very legitimately limit the ability to use these resources for anything but emergency standby power. In fact, only a fraction of existing DER are even interconnected with the electrical T&D system. That suggests an enormous resource, equivalent to about one quarter of the 950 GW of grid capacity, is not used for all but a few hours each year. This could all change with cleaner, quieter, easier-to-permit, emerging technologies such as fuel cell systems that allow on-site generation as well as emergency power. If all of the standby power were grid connected, could earn customer revenues, and could be called upon to support the grid when necessary, the energy-security implications could be great. Moreover, for many high-tech industries, power quality and absolute reliability would be highly valued and worth much more than the energy provided by fuel cells.

While most of the attention focused on fuel cells has been directed toward their use in vehicles, the much higher overall efficiency of stationary fuel cells for combined heat and power (CHP) applications adds considerably to their economic viability. For example, under reasonable assumptions the cost of electricity from a fuel cell with cogeneration would be about one-third less than from one generating electricity alone. Designing CHP systems to best take advantage of waste heat can be a challenge. Should the system be sized to meet the thermal load or the electrical load? To help balance the thermal and electrical loads, should cooling systems shift to gas-fired absorption chillers? Fuel cell technologies are still evolving and it is not clear which will be most appropriate for residential or commercial buildings. Will economies of scale and overall 'hassle-factors' result in them being used primarily in larger buildings? Might microgrids set up for clusters of smaller buildings be viable? What are the implications for the grid if a large number of buildings begin to incorporate CHP systems? These questions require deep understanding of the interconnected system.

Some DG is strictly for generating power. With an annual growth rate of 40 percent for the past 5 years, photovoltaics are the fastest growing on-site DER technology. Three-fourths of the roughly 1 GW of global PV sales in 2005 are destined for residential, grid-connected power generation. The environmental advantages of PVs are potentially great, but the policies required to encourage and properly value their potential contributions are not. The implications for the grid of an increasing fraction of residential power being self-generated from an intermittent source such as this are uncertain and worth exploring in PEEC.

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Electric Energy Storage

Electric energy storage options include battery systems, flywheels, ultra-capacitors, compressed air energy storage, and reversible electrolyzer/fuel cells with hydrogen storage. Energy storage can be used to augment the grid by providing power during times of peak demand and it can also provide ancillary services to the grid by increasing power quality, supplying grid reserves, frequency regulation, voltage stability and reactive power support. There are already a small number of relatively large battery systems that provide some of these T&D services, including a 250 kW/2Mwh vanadium redox flow battery in Moab, Utah and elsewhere a 1 MW, 8-hr peak shaving sodium-sulfur battery used for peak shaving. The Institute's interest, however, will be in storage on the customer's side of the meter, which will focus us on a larger number of smaller, more robust, simpler systems.

Reductions in cost and increases in energy density (Wh/kg) for lithium-ion batteries have enabled these batteries to completely dominate the portable battery market in the last 5 years. The economies of scale that go with their huge market in laptops, coupled with the potential market in hybrid and all-electric vehicles, and enhanced by the potential for a doubling or tripling of energy density, could provide an exciting area for research in applying these batteries in the realm of buildings, cars and the grid.

Energy Storage technologies can also increase the predictability and dispatchability of intermittent renewable energy systems, such as photovoltaics, as well as to shift their supply to times of day when their output is more valuable. With 80 percent of the projected growth in photovoltaics to be located on rooftops, the two places to imagine augmentation of PV with storage is right there in buildings or perhaps in plug-in hybrid vehicles or all-electric vehicles plugged into the grid.

Storage used as uninterruptible power supplies (UPS) can provide customers with greater reliability and power quality for sensitive loads. In addition, the higher reliability of energy supply could translate into a greater use of the programming features of thermostats, and thus to reductions in energy use.

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Integrating the System

The previous sections discussed the possibilities of vehicle to grid systems. Such systems including PHEVs or EVs could easily draw their power from many sources, including home recharging stations.

In addition is the possibility of a 200 ft2, 2 KW rooftop photovoltaic array selling electricity to the grid during the day and buying it back at night to provide the electricity for a PHEV or EV. Right now such systems would not be economically attractive even if the PHEV or EV were readily available.

What could these prospects mean for the environment and for the grid, if the technologies and systems were to be developed and the cost of PVs drops sharply? How would a well-to-wheels analysis of PHEVs and EVs evaluate the carbon emissions compared to conventional vehicles as a function of the location in the country where power is generated? What would be the implications of adding load to the night-time power demands? Would that off-peak electricity be generated with idle coal plants, new combined-cycle gas plants, or a new generation of nuclear reactors?

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Stanford's Living Laboratory: The Green Dorm

Stanford is on the threshold of having the capability to evaluate the physical realities of some of the research questions raised as we consider possible synergies between buildings, the grid, and vehicles. A team of professional architects and engineers, working with university faculty and students, are currently developing the design for a 47-bed 'row house' green dorm on campus. The vision is to have this become a 'living laboratory' that will evolve with time. Our goals include on-site generation of as much electricity annually as will be used by the dorm, with a hope that we can also make it a net-zero-carbon building.

The current design includes about 15,000 ft2 of residential floor space with a building envelope that will emphasize energy efficiency, green materials, natural ventilation, daylighting, passive solar heating, building-integrated photovoltaics, and a solar-thermal system for hot water. In addition, a 5000 ft2 laboratory will be set up to evaluate the performance of a number of energy systems to provide heat and power for the dorm, as well as heat recovery from hot and warm waste water, on-site grey-water treatment and potentially, with time, complete wastewater treatment. We hope to be able to do side-by-side testing of a number of heating, electricity generation and hot water systems including possibly a geothermal heat pump, a fuel cell system, and perhaps a Stirling engine. We also hope to include test facilities for vehicle-to-grid systems and perhaps will provide on-site hydrogen refueling for fuel-cell vehicles.

Our design approach is to make everything in the laboratory be as flexible as possible so that we can test, evaluate, and improve a range of technologies that may not be ready for commercial application when first installed. When a new model of something comes along, we want to be able to replace the old with the new without having to make major retrofits to the structure, the heat distribution system, the wiring or the plumbing.

The green dorm will provide a physical complement to the already strong policy side of Stanford's work on energy efficiency and new energy systems. A faculty search has been approved for a senior-level engineering position in energy efficiency and renewable energy. In addition, we are located very close to the Electric Power Research Institute, where policy research in DER, PHEVs and the grid has been well underway for some time. PEEC will provide the framework to help 'glue' all of these exciting new efforts together.

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