Your questions answered: Combined heat and power

The May 31, 2017, webcast focused on cogeneration principles including combined heat and power (CHP) and combined cooling, heat, and power (CCHP). Unanswered questions were answered in after the live event.


Christian Mueller, senior sales engineer at MTU Onsite Energy, tackled unanswered questions from the May 31, 2017, webcast on CHP applications and best practices. The webcast focused on cogeneration principles including combined heat and power (CHP) and combined cooling, heat, and power (CCHP). Common factors that influence the success of most cogeneration projects—such as payback period, spark spread, fuel source analysis, and thermal and electrical considerations—are discussed in detail. Examples of successful cogeneration projects from around the world are provided, highlighting the unique attributes of each project. Best practices to consider when evaluating and planning a potential cogeneration project are also identified.

Question: How big of a replenish lube-oil tank is needed?

Christian Mueller: This depends on the engine size, fuel type, and expected run hours of the unit. For continuous operation, we typically have external tanks between 250-400 gal on a 1 MW to 2 MW genset.

Q: How often do you get a full exchange of the oil?

Mueller: This depends on the engine size, fuel type, and lube oil system design. Typically, 1,500 hours or more.

Q: Please discuss the dangers of over-cooling exhausts, which can result in corrosion of the flue.

Christian Mueller, senior sales engineer at MTU Onsite Energy tackled unanswered questions from the May 31, 2017 webcast on CHP applications and best practices.

Mueller: This is especially a problem in biogas installations, which have H2S in the gas. This can cause corrosion in the exhaust if the exhaust is cooled down to where condensation is expected. For biogas installations, we recommend an exhaust temperature no lower than 356°F.

Q: Have you been able to find a reliable, in-line continuous H2S gas analyzer for lower H2S concentrations (under 300 ppm)?

Mueller: Yes, there are multiple manufacturers of continuous H2S analyzers. Landtec makes a nice portable system, and a nice fixed unit is manufactured by Delta Instruments.

Q: How many run hours per year in this application?

Mueller: CHP units can exceed 8,000 run hours per year. The engine has no run hour/load limitation with a continuous rating (3A).

Q: Discuss pros and cons of adsorption chillers versus absorption chillers for CCHP.

Mueller: All the chillers we have worked with for CCHP have been absorption type chillers.

Q: With multiple gensets, say more than four, is it more cost-effective to use a combined HEX for the exhaust gases?

Mueller: Yes, it can be more cost-effective to combine to combine exhaust. However, care must be taken that no exhaust from a running engine back-feeds into an engine that is shut down.

Q: Are your engines wet sleeve?

Mueller: We use wet cylinder liners.

Q: Do you also recommend using a heat transfer fluid such as Dowtherm for heat recovery?

Mueller: The engine heat recovery circuit is filled with a water/glycol mixture. The building side is typically treated water.

Q: Do you offer or know of a low-cost siloxane removal system?

Mueller: Yes, we can offer gas conditioning for different biogas streams.  The cost of the system is based on the gas flow and level of contamination.  We will always review the initial capital cost of the gas conditioning equipment versus the operational cost and select the best solution for the project.

Q: Are any of these systems suitable for multifamily applications? Such buildings typically have high domestic hot water and heating demand but relatively lower electrical loads.

Mueller: Typically not, because the load of such installations is still too small. It would need to be a larger condominium or housing unit with high sustained loads

Q: No redundancy? What about a hotel?

Mueller: CHP typically is not designed to be the primary backup system. In case the CHP unit is down, the hotel will have the utility to pick up the load. In case the utility is down, the CHP can be used as a backup in combination with diesel generators for life safety. But few customers will pay to install CHP redundancy.

Q: What unit do we need to use to capture landfill gas? and what would be the smallest capacity unit to produce for a Landfill?

Mueller: Landfill gas capture systems would be provided by specialized companies. Typically, most landfill sites have generators that are greater than 600 kW as return on investment is too low with smaller units, which are more expensive in terms of $/kW.

Q: 1 kW cooling to 3.515 kW cool = 1 Tr. for 1 MW, are you saying we can obtain up to 3515 Tr.? Seem like it’s high.

Mueller: 1,000 kWe = 1,000 kW cooling. 10,00/3.515 = 284Tr.

Q: Would a school that needs cooling, and an outdoor pool that needs heating be a good candidate for a CHP absorption chiller installation?

Mueller: Yes. It would, however, depend on the size of the school and load profile as well as local electric rates.

Q: Why is the engine preferred to a gas turbine, and is it true that the gas turbine should run 24x7 at full load to get its expected design life?

Mueller: An engine offers more flexibility in terms of turn-down ratio and fuel efficiency (for simple cycle).

Q: I agree about the efficiency generated through CHP, but I am concerned about the end user now being burdened with the maintenance equivalent to “utility” company efforts. Can you speak to this burden and offer some other paradigms that would allow the end user to harvest most of the savings but not get burdened with this level of maintenance that would typically require special staff?

Mueller: The end user can sign up for a long-term service agreement with a servicing distributor of the engine brand. The distributor will then perform all maintenance at the site with his own people. The end user will pay a flat fee per operating hour. This not only takes away the burden of having his own maintenance staff, but also makes operating costs predictable. This is a very common arrangement for CHP systems.

Q: What is considered a good spark spread between a natural gas rate and an electric rate to justify cogeneration?

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