Combined Heat and Power (CHP) is a proven approach to save energy and money by locally generating electricity and useful heat at the facility. Generally, CHP systems operate in parallel with the electric utility, with the CHP providing baseload power while the utility supplies the peak loads. A key advantage to generating your own power onsite is the ability to continue generating power during a utility outage, and more of our clients are taking advantage of this feature. So what are the key considerations when using CHP for not only everyday energy efficiency and savings, but also providing highly resilient power during a utility outage?
Identify Maximum Electric Load
This will sound obvious, but it’s essential to understand the maximum electrical load that will need to be supplied during the utility outage. CHP units have maximum power output ratings, and if the building systems try to pull more electricity out of the CHP system than it can provide, the CHP system will be overloaded and shut down. Because it is typical to size CHP systems at less than the peak loads of the facility – to optimize CHP utilization – it is often necessary for a set of essential, critical loads to be identified that are less than the CHP capacity. Automatic load shedding or a dedicated load panel can be used to limit the power draw accordingly.
Understand Minimum Electric Load
Understanding the minimum electric load that the CHP will have to supply during an outage is also important. While CHP systems can reliably operate at any power level, keeping them above 50% of their rated output is best. The concern is that low power operation results in lower pressure in the engine cylinders, allowing slippage of oil across the rings and into the combustion chamber. The result is carbon deposits in the exhaust system, including exhaust valves, heat recovery heat exchangers, and silencers. This carbon deposition is a cumulative process, so short periods at low power are not an issue. MWM, for example, suggests that operating down to 30% power for up to 2 hours can be managed if followed by an equal period running above 70% power. Because emergency conditions do not generally persist for long periods, the minimum load may not be a big concern. But projects that are completely islanded from the grid, or expect long periods of islanded operation, need to pay careful attention to minimum electrical loads.
Understand and Manage Transient Step Loads
Natural gas engines used in CHP systems are designed for maximum efficiency and durability rather than for rapid transient response. Most engines can only handle steps up to 20 or 30% of rated power, as shown in the example ramp rate chart below for the 800 kW MWM TCG3016 V16. This limitation is not only for up-transients, but also down-transients as electrical loads turn off.
Given these limitations, it is essential to understand the load step characteristics of the critical electrical loads. For example, what are the largest motors that may turn on/off? In many buildings, the typical suspects are large electric chillers and elevators. For industrial facilities, large electrical loads may be turned on at the beginning of a shift, including lighting and processing equipment.
To manage large step loads there are many technical solutions that will have additional energy efficiency benefits. VFDs are used on large motors to limit inrush current and peak power demand during startup. Lighting control panels are used to stage the lighting turn on/off cycles. And manual procedures are used to manage the start-up / turn-down processes of many industrial operations.
Determine Transition Method Upon Loss of Power
Because CHP systems typically run continuously, they will be operating when the utility outage occurs. There are several options for what happens then, from fully automatic to manual, depending upon the facility’s needs. At one extreme, the CHP and electrical system are designed to continue to operate without losing power to the critical loads. This is accomplished by immediately opening a breaker that isolates the CHP and critical loads from the utility system. The control system on the CHP can coordinate this, but the facility switchgear must include a motor-operated breaker and the proper controls elements. Such an automated system may also require a load shedding and/or load management system to ensure the CHP is not overloaded.
At the other end of the spectrum is an entirely manual process in which the CHP system is allowed to shut down when it detects a utility outage. Facility personnel would then manually isolate and switch loads to the CHP unit, possibly manually reducing loads by opening panel breakers for non-critical equipment. They would then start up the CHP unit to power the critical loads. This solution is the most economical and can be the right system for buildings such as schools and government buildings that use the CHP to provide power and heat for the community during a natural disaster, as an example.
Natural Gas CHP vs. Diesel Standby Generators
It is helpful to consider the differences between high-efficiency natural gas engines used for CHP systems and diesel engines typically used for standby emergency generators. Diesel gensets meet NFPA 110 code requirements for emergency power systems, which requires start-up within 10 seconds, whereas CHP systems typically take about 3 minutes to start-up and reach full load. In addition, diesel gensets typically operate about 100 hours per year with high responsiveness for supporting large step loads, while natural gas engines for CHP systems are designed to run continuously with high-efficiency for 20 years or more. Ultimately, both systems have a role in the overall energy resiliency of a building: diesel gensets for code-compliant emergency power, and CHP for resilient, “discretionary” power for continuing operations through a utility outage.