Direct Current Microgrids: The Next Big Thing
By far the most exciting development in building construction today has little to do with building construction; it has everything to do with how we power our buildings. Passive House techniques have perfected how we assemble the building envelope and how we heat, ventilate and cool buildings. Through these techniques we have been able to reduce heating and cooling demand by 90% from where we were at the turn of the millennium. But these techniques only deal with energy demand; they do nothing to affect energy supply. To get to affordable zero energy and energy positive homes, the building industry now has to tackle the supply side.
Alternating current, advocated one hundred years ago by Nicola Tesla and George Westinghouse and set as the standard for the transmission of electrical power, stands in the way. We are moving to a direct current (dc) world. Consider this:
Photovoltaic panels generate energy in the form of direct current. Battery systems store energy in the form of direct current. Your refrigerator and the motors in all the new high performance mechanical systems are driven by direct current. LED lighting uses direct current. Your computer, your phone, your TV and other entertainment devices and your laptop all use direct current. Yet every one of these items requires a rectifier/power supply (the little box you plug into the wall when you charge your phone, for example) to convert alternating current to direct current. Every time that conversion happens, energy is lost in the form of heat. When we are talking about on-site generation and storage of energy, those cumulative losses can become significant – as much as 50% according to Dr. Rajendra Singh at Clemson University.
Onsite energy storage in batteries is growing by 40% a year according to Navigant Research. Yet think of the path of electrons from your rooftop solar panels: The first thing they do when they leave the roof is get converted to ac current so they can move across your house to the battery storage system. There, they immediately get converted right back to dc to be stored in the battery. Then they leave for, let’s say, your refrigerator and they get converted again to ac to get to the fridge. Once there, they get converted yet again so that they can drive the variable speed direct current motors of the refrigerator compressor. That is four conversions! Each with a significant loss of electrons.
Now what if the wiring system in the house were a 380 volt direct current system? It is easily doable since direct current can be transmitted in standard household wiring. Then there may be no need for any other conversions for 380vdc power devices or the only conversions necessary would be stepping voltage up or down as/if required by the motor or appliance – far less of an energy loss. Today roughly 1/3 of all electrical items in the home are native dc. With very small changes, 2/3rds or more could be native dc, and as mentioned before, solar and battery, as well as wind systems, are all native dc.
The only thing that stands in the way of changing the “language” of our household energy from dc to ac is inertia. But things are changing quickly. Here’s how:
- Just last week the National Electrical Code announced a chapter covering dc microgrids in its 2017 code book.
- In September Mitsubishi announced that they are now going to make a dc powered mini-split heat pump (like those we already use in our passive houses). In addition they have announced they will be producing a dc powered energy recovery ventilator.
- The EMerge Alliance is in the process of creating international standards for dc voltage within multiple use cases.
- CLASP is pioneering the development of dc powered appliances, both for the western markets as well as the third world where energy efficiency can make a difference between being able to purchase basic necessities or not.
- Already there is a broad range of dc lighting products available and more are coming onto the market each year.
- Elon Musk, Tesla and Solar City are leading the charge toward dc. It makes sense, since the car and the Powerwall are both native dc. Tesla is expected soon to make the Powerwall-to-vehicle connection a direct dc connection eliminating the inverters now in place. Ironic that a company called Tesla is moving toward dc, when Nicola Tesla is the main reason we live today in an ac world!
So where is this all going?
With the ability to create a nanogrid – solar power with battery storage and energy management software—homes can now be “islandable”, that is, self sufficient during power outages; or be designed to be totally off-grid. They can also sell energy back to the grid during peak load hours. These attributes are priorities for government and utilities, both of which are increasingly concerned with the resilience of the grid as well as leveling out peak loads to eliminate the need for additional power plants.
Linked together in neighborhood microgrids, these individual nanogrids become virtual power plants, where energy is made and traded. Such virtual power plants are now a fact of life in Texas and California. With the growth of battery storage capacity (tenfold from 2014-2015 in the US), a new transactive energy economy will be born. Navigant Research projects that $2.1 billion will be spent annually on implementation of virtual power plants by 2025. See their chart below of the current growth of battery storage.
How can we as architects and engineers ignore this trend and not change how we wire our new homes? In our practice we are now making all of our new homes as dc-ready as possible so that in ten years when the shift has happened, conversion will be simple. Within that time period standardization will have occurred among product manufacturers and among UL, NEC and other regulatory groups. Because of the work of The EMerge Alliance and other national and international standards organizations, 380v dc will be the new 120v ac, and the nice thing about that is that today’s household wiring can carry that voltage. All we need to do at this point is to provide the circuits necessary for battery storage and solar production, as well as dedicated dc and ac outlets.
Right now we are working to develop a case study hybrid dc microgrid in a new project in Maryland. The proposal involves six contiguous sites and six identical modular zero-energy houses to create the first sustainable dc powered community. Each house will have its own nanogrid. The six houses will also be interconnected through a dc microgrid. With the help of Terry Hill, who sits on both the PHIUS and EMerge boards, we are working now to assemble products, expertise and funding for the case study microgrid design and implementation. While many technical papers have been written on the advantages of direct current, and small academic demonstration projects have been set up, no true real-world hybrid dc microgrids have yet been built to put theory into practice. This project will do that at a residential scale.
To learn more about the potential and rationale for direct current, CLASP has put out an excellent white paper on the subject. There is also an excellent article and references on virtual power plants in Microgrid Knowledge.