Your questions answered: Renewable energy systems

Electrical engineers have several items to consider when interconnecting renewable energy production systems

By Sean Avery and Benjamin Skelton June 9, 2022
Courtesy: Consulting-Specifying Engineer

This webcast on Critical power: Renewable energy systems on May 24, 2022, covered many topics, and additional questions are answered here.

Whether designing a renewable energy production system to connect into an existing building’s electrical system, or designing a solar-ready facility where only a means to interconnect a future renewable energy production system is provided, the details of the interconnection can have a major impact on the design, cost and schedule of a renewable energy production system project.

Subject matter experts:

  • Sean Avery, PE, LEED AP, Electrical Engineering Leader, Principal, DLR Group, Seattle
  • Benjamin Skelton, PE, CxA, LEED AP, President, Cyclone Energy Group, Chicago

Approximately how much of net zero building goals can be achieved via roof top solar? In general, how to integrate solar/photovoltaic, inverters, etc. into the building electrical distribution system?

Benjamin Skelton

Benjamin Skelton

Sean Avery

Sean Avery

Skelton: 100%! In fact, projects seeking a net zero goal should plan for excess amounts of at least 5% to account for changes in weather year-to-year. And over a 25-year review, I’d suggest at least 110% over production on the baseline year.

What considerations go into selecting a connection point as being downstream or upstream from the service disconnecting means?

Avery: Typically system size. The smaller the system relative to the service size lends toward a load-side connection. As the system approaches the ampacity of the service, a line- or supply-side connection is typically used. In some cases, line-side connections just aren’t feasible, in which case we request reducing the main breaker (based on metering data) to allow for an increased load-side connection.

The cost that you showed for the different mounting types, did that include material and install cost? We have seen more between $1.60/watts to $1.80/watts for roof ballasted systems in the Midwest for both.

Avery: The costs I showed are what I have seen recently on the West Coast. They do include some assumption for structure.

Battery energy storage systems may be considered as part of the renewable system. Courtesy: Consulting-Specifying Engineer

The collapse of the Texas grid was a result of a failure to winterize the generating assets coupled w a normal maintenance outage for nuclear generating assets. How can the mission critical facility ensure that their renewable generating resources are protected from similar events? Grids are complicated and sensitive.

Skelton: Mission critical should be as separated from variability and instability to the greatest extent possible. Battery storage and uninterruptible power supplies can be integrated with renewables such as solar, however given the dependency on sunlight, are not a reliable replacement for generators.

I’ve heard about mini-hydro, in streams or even pipes inside a building. Did that ever take off?

Skelton: I’ve not heard or seen and recent installations, I’d presume no, it’s not yet a viable technology, but microgeneration in general is a highly likely new tech to emerge in the coming years. Integration into home/building electrical systems is the biggest barrier to overcome. If the grid changes from alternating current (AC) to direct current (DC), things would be much easier.

What would be a rule-of-thumb cost per panel for the three types of panels?

Skelton: Included in the presentation and it was installed cost, not panel cost.

What’s the thin film life expectancy?

Skelton: All panels fall of in production throughout their useful life. Thin film diminishes the fastest and the rate depends on the film media used. It can be anywhere from 7 to 20 years.

I would not consider small wind as a significantly viable source for building/site applications due to cost and concerns about: sufficient wind/power production, visual issues, vibration, maintenance and insurance risks.

Skelton: Agreed. Wind also has issues with bird migration flight paths, which are complicated to navigate.

I like solar, but I several of the solar units I have seen in Ashville, North Carolina; Orangeburg, South Carolina; and Greenville, South Carolina, have replaced areas that were forested causing removal of wildlife habitat and increasing a requirement for land maintenance. Are there any organizations that are keeping metrics of solar installation that are tracking the loss green spaces and calculating the true energy gain, carbon reduction and negative or positive environmental contributions?

Skelton: Not that I’m aware of, however there is significant research into solar agriculture. Growing crops under solar fields that shows potential for renewable energy, more consistent and higher yield crops and higher overall carbon reduction impact.

Biggest challenge is really fire protection requirements for roof top units. Do you agree?

Skelton: Dealing with authorities having jurisdiction is always the most fun, however grid interconnection with utilities is right up there.

Do you max the direct current (DC) or alternating current (AC) run?

Avery: I am assuming this relates to distance. There is no perfect answer, but generally there are a lot more conductors (although smaller) on the DC side. I have looked to see if I could take advantage of a higher voltage on the DC conductors, but it usually works out better from a conduit/cable/cable tray scenario to keep inverters closer to their modules and max the AC run.

Is home solar ever going to be profitable?

Skelton: In some locations, it already is due to incentives, rebates and tax credits. With the cost per watt installed dropping, home solar will be much more common, however utilities need to have a better understanding of how it will impact the local grid as stability is a delicate balance.

Are hydrogen fuel cells renewable?

Skelton: Fuel cells in general are storage devices and hydrogen is a basic finite element that is not renewable.

Are these costs inclusive of supporting equipment like disconnect, inverters, etc.?

Avery: Yes, for a basic system without additional control systems, however it will vary from area to area and by utility.

Energy storage at the customer side allows for the voltage, VAR and frequency support that will be needed with a grid comprised of primarily renewable/variable generating assets. Grid interactive load management changes the equation giving the consumer a greater financial stake in the equation. Please rank the measures that customers can implement starting with those that have the greatest return on investment and net present value.

Avery: This really depends on where you are and how your utility rates are structured. Rates that are structured more around energy (kilowatt-hour) will benefit more from solar or other generation systems. Rates that are structured more around time of use and/or demand will benefit more from storage if peak demand does not coincide with solar (or other energy) production.

Should you not perform a complete energy reduction project before a solar array installation? Your solar project would be less expensive and smaller footprint.

Skelton: You should perform an energy reduction project at the earliest potential time regardless of solar. If you over produce, often that can be revenue generating.

What happens when the roofing material needs to be replaced? Disconnect, remove and replace? Warranty?

Avery: You will need to remove sections of the array and associated equipment temporarily. Roof anchors will probably need to be replaced (to match roofing material) and rewelded. Most projects I see wait for a roof upgrade or go onto a relatively new roof.

Renewable energy generation recently surpassed coal and nuclear electricity generation. Courtesy: U.S. Energy Information Administration

Does the EPC and PPA costs listed in the presentation include taxes and maintenance as well?

Avery: The rates I showed typically involve all of that, yes.

Would it be best practice to charge a BESS overnight or during peak PV production? Or does this depend on the rate structure? Example given had an overnight BESS recharge.

Avery: The example I gave probably should have included a daytime charge from solar given the significant back feed. When I created this, I initially looked at the two systems separately and, without solar, nighttime charging would have been better. The best charging time depends on energy resources and utility rates. In short, minimize charging expenses.

Any experience with reverse cycle chillers as the electric heat source?

Skelton: They will be the near-term future if not the long-term future of heating and cooling buildings. Current technology has limitations with heating temperatures and difficulty above 120ºF. This makes use in retrofit building often difficult but not so much in new designs. A2L and natural refrigerants like CO2 have the potential to revolutionize heat pump chiller use.

Latitude versus tilt angle?

Skelton: There are tilt angle advantages to panel efficiency. However, it depends on the size of the array before it makes a tangible difference toward the bottom line.

Are there incentives or issues coupling reenable energy production with electric car charging stations?

Skelton: Not that I’m aware of. Many states offer incentives as do a lot of cities for electric vehicle charging stations.

For a small sewage pumping station, do you think the solar panel industry is ready to provide a more reliable power source during power outages?

Skelton: The battery storage industry is the group to talk to on this. If you know the outage threshold and interval, solar can do its thing with a properly sized battery storage system.


Author Bio: Sean Avery, PE, LEED AP, Electrical Engineering Leader, Principal, DLR Group, Seattle Benjamin Skelton, PE, CxA, LEED AP, President, Cyclone Energy Group, Chicago