Your questions answered: Critical power: Combined heat and power systems

Cogeneration systems, often referred to as combined heat and power (CHP) systems, generate both electricity and thermal energy. As they become more common in the United States, engineers must understand the nuances and design strategies for successful application.

Rodney V. Oathout, PE, CEM, global energy services leader and principal at DLR Group in Overland Park, Kansas, responded to questions not answered during the live event entitled Critical power: Combined heat and power systems held on March 7, 2019.

Question: Can cogeneration systems be used as a life safety backup power source?

Oathout: Maybe, and it depends on a lot of different issues. Advice on places to start:

• Thorough code review to determine if your situation even has a chance.
• Early meeting with the authority having jurisdiction (AHJ). AHJs are commonly fair with early notice and explanation.

Question: What are the limits of exhaust gas capture?

Oathout: The same as usual: hot enough gas to matter without generating too much condensate.

Question: What about microturbines combusting biogas? Track record?

Oathout: It’s a dream come true for most of us. Research suggests too few applications to cast judgment. Remember, biogases can be tricky. Be mindful of metallurgy, dew point temperature and emissions in situations like this.

Question: Are there options for hybrid liquid natural gas/diesel on the same prime mover?

Oathout: Gas turbines are good candidates for this approach. This is not a common option, so we suggest reaching out to the equipment manufacturing community to see what is possible.

Question: Any guidelines on physical limitations for steam export from cogeneration installations? Most steam usage typically is onsite, but knowing typical export limits would help define potential for cogeneration proposal applications.

Oathout: Our experience is the more thermal users the better, especially if they are nearby and have similar thermal energy quality requirements. The biggest challenge in combined heat and power (CHP) system is finding a use for the thermal energy.

Question: Can you parallel microturbines and gas turbines together?

Oathout: This is a good strategy, especially if there are varying thermal loads. The key when mixing technologies includes:

• Are the thermal characteristics similar (temperature, pressure, flow rate)?
• Is the power generation useful?
• Can the project afford it?

Question: Are northern climate zones preferred?

Oathout: Not necessarily. The added heat requirement in the winter months are useful, but there are many other factors that apply. A life cycle cost analysis is your friend.

Question: You talked about shutting down the CHP system when you use utility power. What about just disconnecting from utility upon utility power loss and using the CHP as an optional backup system, but not a life safety system?

Oathout: A couple of things to remember, CHP systems may or may not be applicable as a life safety system. There are

Question: Six kilowatts seems very small for a project of this size. What other factors were considered?

Oathout: Six kilowatts happened to be the electrical generation capacity if the equipment considered in the case study. The consumption of the building as much greater.

Question: Why aren’t the temperatures the same at 0 and at 24?

Oathout: The performance charts shown in the presentations were simulations. In real life, the CHP equipment would have to shut off for a period of time to manage the temperature of the storage tank.

Question: How do you modulate or control three parallel CHP units to really fine-tune the water temperature? And how you controlled it through return water temperature?

Oathout: First, the final design we proposed used two CHP modules. The control of the CHP modules was based on the temperature of the storage tank. The size of the storage tank “think buffer tank” is key in a successful solution.

Question: What modeling tool used for domestic hot water (DHW) tank temperature to kilowatt hours?

Oathout: We are not aware of a software package available to perform these calculations. Hand calculations were used, then transposed into Microsoft Excel for automation purposes.

Question: Please explain the electrical savings between site generation and off-site purchase again. Is the difference the losses of distribution?

Oathout: Yes. These losses are significant, especially if you consider greenhouse gas emissions. I encourage everyone that reads this to understand the fuel and emissions dynamics associated with purchasing energy from the grid versus generating it on-site.

Question: In your case study, you are generating too much electricity. What are you doing with it?

Oathout: The electricity produced in the case study example is mostly being consumed locally. The location of the facility has net-metering provisions, so the electricity is economically transportable.

Question: How is system controlled during a power failure?

Oathout: In this case, it is turned off. Please review and understand anti-islanding rules when design any on-site power generating system.

Question: Are CHPs used with refrigerated warehouses and the recharging of forklift trucks during the slow traffic hours?

Oathout: This might be a good application for CHP technology.

Question: In considering the value of onsite power production in the case study, did the grid equivalent kilowatt hours include the lost opportunity of heat recovery? It’s not just line losses, right?

Oathout: Heat recovery is not generally part of this calculation. The inefficiency associated with losses from transporting power across the country generally defines the difference between source and site energy.

Question: Could you please clarify the reason why source energy as you cited are so much higher than site energy (i.e. 85,000 kilowatt hours versus 256,000 kilowatt hours)? Which state’s data does that example refer to?

Oathout: We suggest that you research and understand the difference between source and site energy. This is a fundamental topic when understanding the value of CHP systems and microgrids.

Question: In the case study, was electricity exported to the grid at any time of the day?

Oathout: It is possible. Our analysis identified a small part of the electricity would be exported. The amount was so small and favorable export rules suggested that it was a non-issue.

Question: You said the CPH connected to utility will have to shut down if the utility is down. Do you mean just disconnected from the utility?

Oathout: Yes. The point I was making is there are codes and utility-based rules that require on-site power generating equipment to be disconnected from the grid in the event of a grid power failure.

Question: The most attractive feature of a CHP is to be able to normalize energy cost. For example, by locking in natural gas for five or 10 years in the competitive energy market compared to relying on unpredictable rising electrical rates. Also, big benefits in peak demand savings. Are these benefits only considered in large industrial applications only instead of smaller commercial sizes? What is considered the breakpoint?

The majority of CHP equipment manufactured today has application for larger industrial and even larger commercial system designs. The manufacturer of equipment for commercial buildings continues to be a work in progress. The “breaking point” is undefined or, at a minimum, undetermined today.

Question: Is a power purchase agreement (PPA) where a third-party owns the CHP equipment always the best choice if it is available?

Oathout: Every situation is different, PPA can be beneficial for owners that cannot benefit from the favorable tax judgment.

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