Your questions answered: Microgrids 101

During the April 30 event Microgrids 101: Combining multiple power sources for maximum efficiency and uptime, several questions from the audience were not addressed. Responses to these questions are here. 

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Question: You seem to present biogas as flare gas. What about syngas using pyrolysis from municipal solid waste, tire derived fuel, waste vegetable oil, etc.? Those feedstocks are a pipeline of stored-energy fuels. Just in time waste processing using distributed generators can eliminate large storage dumps.  

Christian Mueller: Pyrolysis gas is typically not suitable for combustion in reciprocal engines due to its composition.  

Question: What percentage of microgrids use reciprocating engines for combined heat and power and what percentage use turbines for combined heat and power and why?  

Christian Mueller: Most installations less than 10 megawatts will use reciprocating engines as they are more flexible in operation and have higher fuel efficiency compared to turbines. Turbines are used in larger plants or where production steam is the priority  

Question: Discuss mixing solar and wind with reciprocal engines for microgrids.  

Christian Mueller: Reciprocal engines are a perfect fit to complement solar and wind. If electricity generation from wind or solar drops, reciprocal engines can be started to produce power. We will talk more about this in Microgrids 201: Integrating renewables and battery storage into your power solutions. 

Question: Is there an explanation as to how wind power “could reduce severity of storms”?  

Brian Ponstein: Please see this article for more information 

Question: Any thoughts regarding the use of fuel cells?  

Christian Mueller: Capital cost for fuel cells remains very high compared to other technologies. Unless there are external factors such as government incentives, fuel cells see limited use in micro grids compared to other technologies.  

Question: What about battery recycling?  

Christian Mueller: Typically, the battery supplier offers a recycling program for the batteries at the end of their life. 

Question: Power quality usually refers to the wave form and phase relationships on the alternating current lines. And the presence or absence of transient spikes/surges. What elements of a microgrid would manage the transients produced by engaging and disengaging the different generators, storage and loads to maintain acceptable power quality?  

Christian Mueller: Power quality requirements are site specific and are taken into consideration when choosing the mix of generation assets. The microgrid controller will  

Question: How do you balance reactive power?  

Brian Ponstein: In the examples spoken about in the webinar, the reactive power is controlled via the voltage controller on the alternator. The information is gathered within the microgrid controller and then sent to the resource(s) best used to address the reactive power. Then the controller for that(those) resource(s) adjusts the settings of the automatic voltage regulation to properly share reactive power as demanded by the Microgrid controller. 

Question: What design/feasibility software do you use?  

Christian Mueller: MTU uses the HOMER tool to model microgrids 

Question: What’s a typical payback period including tax, interest, insurance, etc.? 

Christian Mueller: Every installation is different, therefore a site-specific analysis would need to be done to include all factors affecting payback. 

Question: Can the MTU on-site Energy controls synchronize and share load without the use of a third-party equipment?  

Christian Mueller: Yes, if the genset and battery storage solutions are supplied by MTU. If there are third-party generator sets, then custom engineering will develop defined interfaces to allow for this functionality.  

Question: How has the restriction on greenhouse gas emission impacted the usage of natural gas/diesel genset as a microgrid?  

Brian Ponstein: As noted in the webinar, emissions are one of the first parameters to address when designing a microgrid. Generally, emissions are something that have to be looked at from a systems approach for each site and what is the best solution for the client. 

Question: Do electric utilities have connection requirements for battery use? If so, what do utility requirements tend to look like?  

Christian Mueller: Most utilities have interconnect requirements for battery system, just as they do for generator sets. Our recommendation would be to inquire through the local utility to get this information.  

Question: For the 2-megawatt photovoltaic system, how much space for battery storage needed?  

Christian Mueller: This depends on how much of the electricity generated by the PV needs to be stored and how much is consumed. Webinar 201 will talk more about battery storage. For reference purposes, a battery storage system with 2.000 kilowatt peak power and 1,000 kilowatt/hour capacity would come as a 40–foot ISO container.  

Question: What’s the renewable aspect of the hospital example? And is the biogas considered renewable in the water treatment plant?  

Christian Mueller: The hospital in this case did not have a renewable aspect. Yes, biogas is considered to be a renewable fuel.  

Question: What kind of coordination had to be done with the utility to create a storm preparedness mode?  

Brian Ponstein: First and foremost, regulatory compliance must be met from both the federal and regional requirements. Once those areas are addressed the requirement for the connection to the local utility need to be addressed 

Question: Is net metering typically included to ensure you are getting value for kilowatt-hour you are producing? Or how do you otherwise match your production to your load to ensure you are not overproducing/wasting energy produced?  

Christian Mueller: Yes, most installations are behind the meter with net metering done by the controls to ensure that the generation matches site load and no power is exported.  

Question: How do you account for load power-factor when sizing in kWe?  

Brian Ponstein: Typically, the generator sets are sized for continuous operation thus they have 100% load factor capability.  

Question: How is engine maintenance handled if only 4 units provided and 4 required for load?  

Christian Mueller: Maintenance is done during times when the plant load is low, eg. weekends or nights.  

Question: When speaking of military applications, are you talking about static locations like a military base or dynamic applications like a forward operating base? Do you have any examples?  

Christian Mueller: Within the U.S., these would be stationary applications at permanent bases.  

Question: Please share your thoughts regarding gas microturbines and how they fit in the lineup of microgrid resources.  

Christian Mueller: Engine driven generators are far more widely used in microgrids as they offer better fuel efficiency, lower cost/kilowatt and cover a wider range of power nodes compared to microturbines.  

Question: Which are the common energy sources used in a microgrid for a remote location (without grid)?  

Christian Mueller: Diesel generators combined with solar and battery storage. 

Question: Are the MTU on-site Energy controllers only compatible with MTU sources or are they compatible with other vendors, like Caterpillar or General Electric Co.?  

Christian Mueller: Yes, it can be done with project–specific engineering.  

Question: Is it possible to export excess microgrid power out into the utility grid?  

Christian Mueller: This depends on the arrangement with the local utility and what rates they will pay. This may also differ depending on what generation asset is producing power as renewable power may have attractive rates for export. Typically, microgrids will however operate behind the meter or in island mode  

Question: What is the common voltage level for the microgrids you are considering to develop?  

Christian Mueller: Most are between 480 volts to 13 kilovolts. 

Question: On slide 30, does the wet-stack mitigation require a dedicated load bank or can the engine just take on more of the load while the other sources ramp-down?  

Brian Ponstein: There are several solutions for to mitigate wet stacking. Being this is a known condition it can be programed into the microgrid controller to adjust load from one resource to another. Thus, the resource that is entering a wet stacking condition can take on a larger percentage of the load. If there was a load bank on-site that could be used as well, but it probably isn’t the most efficient solution for a microgrid, but could be a part of the solution for contingency planning if needed. The final solution would depend on what power generating resources are available at the specific site. 

Question: Do you see natural gas prices rising in the near future to the point where the return on investment will be much more difficult to achieve?  

Christian Mueller: With domestic oil & gas production continuing to grow, gas prices are projected to stay stable in the near future. Profitability will also depend on electricity price developments compared to natural gas prices. Also called spark spread. If both prices rise, it will have less of an impact on ROI.  

Question: Can a microgrid system also be used for emergency power for health care or other facilities that require emergency power?  

Brian Ponstein: Typically, microgrid systems are not used as an emergency power source. The reason for this is mostly due to the cost. However, it may use one power generating resource to reduce the electrical demand from the utility and if the utility fails have additional resources that can be dispatched when needed. For example, a health care facility may use a combined heat and power unit 24/7 as a base load electrical demand and heat recovery. When the utility fails the site may have diesel units (or some other asset) that can be dispatched to allow the facility to operate like normal. 

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