During and after Hurricane Sandy, combined heat and power (CHP) enabled a number of critical facilities to continue operating when the electric grid went down, according to the US Department of Energy, the Environmental Protection Agency and the Department of Housing and Urban Development, which have jointly released a guide to provide information on what factors must be considered when configuring a CHP system to operate independently of the grid.
CHP, also known as cogeneration, is the simultaneous production of electricity and heat from a single fuel source, such as natural gas, biomass, biogas, coal, waste heat, or oil. Instead of purchasing electricity from the grid and burning fuel in an on-site furnace or boiler to produce thermal energy (for heating, cooling, dehumidification, or process needs), facilities can use CHP to provide both energy services – electric power and thermal energy – in one energy-efficient step, says The Guide to Using Combined Heat and Power for Enhancing Reliability and Resiliency in Buildings.
The report says that critical infrastructure assets that should consider using CHP include hospitals and water treatment/sanitary facilities, followed by nursing homes, prisons, and places of refuge, even multifamily housing. Properly sized and configured CHP systems can effectively insulate facilities from a grid failure. In so doing they provide continuity of critical services, and free up power restoration efforts to be focused on other facilities.
In order to operate during a utility system outage, the CHP system must have the following features:
- Black start capability: Similar to the way a car battery is used to start the engine of a car, a CHP system needs an electrical signal from a battery or backup generator located on-site to allow it to start operating when the grid experiences an outage. If both the grid and the CHP system are down and not supplying power at the same time, then the CHP system will need to be outfitted with “black start capability” to begin operating.
- CHP electric generator capable of operating independently of the utility grid: CHP systems use either synchronous or induction generators. Synchronous generators do not need to be connected to an external power grid to operate. However, they need additional safeguards to ensure the CHP system cannot export power to the “downed” grid, which could injure utility personnel or repair equipment. Induction generators require an external source of power to operate. This external power typically comes from the local grid. High frequency generators (microturbines) or DC generators (fuel cells) also need to have inverter technology to ensure they can operate independently of the grid.
- Ample carrying capacity: The size of the CHP system must be matched to the critical loads in the facility. During the design phase of the project, a decision will need to be made on whether to a) size the system for optimal energy and economic efficiency, and designate critical loads that will be supplied during a grid outage; or b) size the system for all of the site electrical requirements and arrange to export power to the grid or operate at partial load on typical days.
- Parallel utility interconnection and switchgear control: When a CHP system disconnects from the utility grid in an outage, appropriate switchgear and controls are required to transition to serve critical loads without overloading the generator capacity. These critical loads must be isolated from the rest of the facility’s non-critical loads, which must be shut down during a system outage through the installed switchgear and control logic.