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.

By Christian Mueller, senior sales engineer, MTU Onsite Energy June 15, 2017

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.

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 24×7 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?

Mueller: $.07 a kW/hr and $4 MMBTU gas will give a reasonable ROI if the system is properly designed.

Q: Did I hear correctly that the expected life of engine is only 8 years, or is that time between major overhauls?

Mueller: Eight years of continuous operation (more than 8,000 hours/year) is typical for the time between major overhauls. At this point, the engine is rebuilt or swapped out with a remanufactured engine.

Q: What about maintenance of the electrical system? I did not see this on your maintenance slide.

Mueller: Scheduled maintenance for electrical systems of the genset is minimal, generator bearings being the only major part. Electrical components, such as sensors or relays may fail over time, but this would be considered corrective maintenance.

Q: What is the effect on lifecycle cost analysis (LCCA) by burning different gases, specifically those that contain siloxanes?

Mueller: Gas quality has a big impact on lifecycle costs, as consumables such as oil or parts like cylinder heads are impacted by siloxanes. Siloxanes always should be removed completely from the gas. Lifecycle costs otherwise become unpredictable due to unexpected failures from silica deposits in the engine.

Q: What is a budget price per kW for a CHP system?

Mueller: This depends on a wide range of factors, so one would need to look at a specific project.

Q: Do you need a condensing economizer to get 90% efficiency?

Mueller: Yes, an economizer helps increase efficiency on a CHP system with a steam boiler. However, to achieve 90% efficiency, the engine jacket water will also need to be used as 50% of the available heat is in the engine jacket water.

Q: Can you briefly discuss micro turbines?

Mueller: Micro turbines are popular where very low NOX emissions are required, for steam production as more heat in exhaust, and for smaller installations in the 50 to 200 kW range. Engines offer better fuel efficiency, lower capital cost per kW, and better performance at part load and high temperature/altitude. Engines also come in wider size range from 100 kW to 10,000 kW. Micro turbines typically are only 50 to 200 kW or multiples thereof.

Q: Could you expand more on the average load dilemma when estimating demand?  Best practices to avoid overestimating run time?

Mueller: The best practice is after doing initial estimates on monthly kW/hr for a specific site is to obtain the 15-minute interval data from the utility, if available, to identify the site demand by time-of-day and peak kW.  This data will allow the proper sizing of the system along with yearly run time. 

Q: Is there a minimum exhaust gas temperature that should be maintained after heat recovery?

Mueller: 248°F for natural gas, 356°F for biogas.

Q: What downfalls come with dual fuel blending systems for IC cogen installations? Ex biogas and natural gas.

Mueller: Engines run best with stable gas composition.

Q: What would be specific challenges of operating a CHP system with pyrolysis gas from wood biomass waste?

Mueller: Pyrolysis gas has low BTU content, low methane number, and often tar as contaminant.

Q: What are typical operational hours for a natural gas engine between complete rebuilds?

Mueller: Eight years with 8,000 hours of operation each year.

Q: Are you familiar with IEEE 1547? Where did this requirement come from? Why is this requirement necessary?

Mueller: IEEE 1547 was designed to standardize interconnection of distributed power generation assets. In practice, each utility still has its own requirements, which need to be considered on a project by project basis.

Q: Do you have a prepackaged CHP system for steam or trigeneration?

Mueller: We offer packaged systems through our distribution partners.

Q: Is there a typical utility cost break/even point (gas and electricity cost) where operating in the CHP mode make more senses than building systems consuming traditional utilities?  Are there areas in the country where CHP makes more economic sense?

Mueller: As a rule of thumb, any time that the cost of electricity is more than $.07/kWh and there is a coincidental thermal demand, you can expect a reasonable ROI on a CHP system.  The U.S. Department of Energy published a report in the spring of 2016 on the Technical Potential for CHP in the United States.  The areas with the most potential were in the Industrial Midwest, North East Corridor, Texas, and California.  

Q: Is water or glycol solution used for jacket water? If water is used, are corrosion inhibitors or additives required?

Mueller: In the engine circuit, a water/glycol mixture is always used. This is to prevent corrosion as well as for anti-freeze.

Q: What is the main purpose of turbo in CHP?

Mueller: To increase the power output of the engine as more fuel is brought to the combustion chamber.

Q: Is there a sizing program that an engineer can use on their own to play some "what-if" games to do preliminary screening of possible applications?

Mueller: The U.S. Environmental Protection Agency Combined Heat and Power Partnership has an excellent preliminary spark spread estimator.

Q: What do you think about meaningful feasibility time as paying off the system itself?

Mueller: ROI really depends on spark spread.  The higher the cost of electricity and the lower the price of natural gas, the more quickly the project will payback.  A reasonable return on an industrial project can be anywhere between 5 and 7 years.

Q: Are there any carbon or gas emissions credits for fossil fuel reduction and emissions produced by the use of utility generated power.

Mueller: The EPA Combined Heat and Power Partnership has an emissions savings estimator.

Q: How do you tell if the CHP system installed at your site has a rich fuel setting or lean fuel setting?

Mueller: We have sites with both lean burn and rich burn engines. Our largest lean burn is 248 kW, all larger units are lean burn.

Q: Do units exists for other gases, such as LPG?

Mueller: Our engines are optimized to run on natural gas or biogas. LPG is typically not a fuel used for continuous operation.

Q: Can a diesel-driven standby genset be used to back up the CHP portion of the system, or can it only provide power backup? Is this economical?

Mueller: The CHP portion, meaning heat recovery, would be backed up by natural gas boilers. Diesel generators would only back up the electrical portion.

Q: How do you compare the unit cost of electricity from a cogen compared to the grid in any state?

Mueller: The National Renewable Energy Lab (NREL) has a levelized cost of electricity calculator.

Q: Can CHP affect or decrease the power factor of the facility?

Mueller: The generator on a CHP unit running in grid-parallel typically can be set to a power factor between 0.8 and1.0. So, a customer can choose to generate reactive power if needed. In an island operation, the facility loads will determine the power factor.

Q: What is the maximum temperature degree and humidity value of the fresh air of the room?

Mueller: This depends on the engine type.

Q: What type(s) of RTU do you recommend for paralleling with the utility company?

Mueller: It really depends on the utility requirements.  It is best to review the utility-supplied interconnection document for the specific protection requirements of your site. 

Q: Do you sell your products outside the United States?

Mueller: Yes, MTU has a global sales and service network.