(Download this schematic at the end of the article)
Designing and installing solar PV in dense urban area is much more difficult than installing 5kW on a 10 pitch shingle roof in the suburbs. The challenges are numerous and include junky roof framing, town homes and apartments with low roof area-to-electrical load ratios, historical committees, and, interconnection to antiquated electrical grids. For this reason, most pure play solar installers and financiers tend to stay away from the hard stuff, they’re still focusing on getting the lowest hanging fruit. However, clients in urban areas are asking more and more solar solutions.
This is the story of how Jamie Leef at the solar division of S+H Construction installed an off grid solar PV system; off-grid in the sense that it did not interconnect with the area network grid in downtown Boston. If you think off-grid/solar power backed-up systems are a thing of the past, think again. They’re likely a thing of the future, as one of nations largest utility IPPs, NRG Energy, is pushing towards solar + battery backed up.
Jamie Leef and S+H Construction are expert builders (see all their awards–>), used to dealing with integrating solar in complex existing systems and structures and is an expert at installing solar in urban areas. If you’re an architect or engineer and need to get advice about integrating solar PV into a complex structure, give Jamie a call.
Continuing Education Free Course
Jamie has created an amazing continuing education course for architects, engineers, and generators. Click here to sign up for FREE COURSE (for a limited time) “Solar PV Design Considerations in Urban Areas for Architects, Engineers and General Contractors”
I had a quick conversation with Jamie about the installation. If you’re a general contractor, roofer, architect or engineer and need help with a difficult project, contact Jamie at S+H Solar here.
Q: Let’s start at the top, what is the reason for having an off-grid system in downtown Boston?
A: There are several sections of Boston, Cambridge in Springfield Massachusetts where the grid is much older.
Normal electrical grids are radial networks where transmission lines come from a substation to transformers and feeders going to several homes. It looks like a hub and spoke.
An area network is an older style of distribution that involves multiple grid feeds, transformers and building service access points. It looks more like an Ethernet network.
The switchgear is much older.
Some utilities have been experimenting in different parts of the country with DG and back-feeding area networks, but engineers are very afraid of this because it’s easy to blow up one of the transformers and the nature of the area network can to shut down if there is any backfeeding.
Q: So, I’m guessing you had a client who wanted solar in an area network?
Our client wanted solar but they were in an area network. They have a house that had a substantial electric use. We were able to design a solar PV system on their property that serves their loads, but doesn’t back-feed into the grid.
We did this by using an off-grid style design where there are batteries fed by solar, and a backup generator. In this case, the backup generator is the grid.
Q: What is the design theory for this application? What loads are you matching? Is the project off grid in the sense that the PV array only charges the batteries, the batteries go to the inverter to supply house loads, but the grid can also charge the batteries?
A: The batteries feed all loads. If they become low, the inverter will switch to the grid until the solar has enough power to supply the load, or until the solar has charged the batteries enough to supply the load.
We designed the system to charge the battery bank in a reasonable period of time so that batteries can meet the average daily loads that is supported by the sun.
In this particular application, there are two parts of the condo each with it’s own electrical service panel. One part includes several rooms and some general, non-essential loads. The second part includes the kitchen, some general lighting, and other circuits that are more critical loads that would be nice to be backed up in an electrical outage.
The nice thing about an off-grid solar system is that you’re actually your own utility. As long as the solar is powering the batteries, you have power.
Q: What were the building and utility integration considerations for building in downtown Boston?
The project is on the top of a 20 story building, so construction is always a little complicated. It’s on the top of a high rise with a ballasted roof, so the wind loads are high which have to be dealt with, but it’s nothing that our roofers are not used to dealing with.
It required a significant amount of negotiation with the utilities for the client to be able to satisfy the Commonwealth Solar Grant program.
There was a design challenge in sizing the solar array and the battery bank for the load that you want, this case the client had to decide how much of their home they wanted to be off-grid. The hourly, daily, and seasonal profile of these loads determined how many modules were needed, and the size and cost of the battery bank.
Another challenge was locating the batteries. Traditional solar batteries are not well suited to be put into living spaces due to hydrogen build, fire hazard, and other issues. We chose batteries very carefully.
The final item is to figure out house wiring. In the case of a renovation, it’s a little easier because the project electrician can place circuits as needed. In the case of an existing home, it can be a little difficult to break out loads in the most optimal way.
Q: You mentioned the largest challenge was getting the utility to sign off on the project. Why was this a challenge?
It’s easy for a solar installer to say “my system is not going to back-feed”. It’s a different challenge for a solar installers to sit in a meeting with 6 utility engineers that and PROVE that the UL 1741 listing for inverters is a guarantee that they can live with and guarantee that there will be no backfeeding. These are engineers that are very familiar with the nature of the electrical grid. This required us to bring in PE and do some pretty intense negotiation.
The reason we need a letter from the utility is to prove to the Commonwealth Solar rebate program that you can get the “interconnection agreement” and access grant funding form the Renewable Energy Trust. They need to be grid tied to use their grant money, which comes from the Renewable Energy Trust.
In this, it’s connected to the grid, but it’s not interacting with the grid. There was no regulatory structure for what we were doing, so we had to create a structure.
We invented a new category for a non-interconnected, utility oriented system. In order to get this, we had to convince the utility engineers.
Q: What were the lessons learned and what is your advice for architects, engineers and property owners that might live in a radial network and want solar. If someone called you and wanted advice, what would you tell them?
The cost of the system with batteries is higher than the cost of a normal grid tied system per kW because of the high cost of the batteries.
However, we have many clients that want some kind of back-up generation in their homes. In the suburbs, people will get a transfer switch and a generator. People like having backup power.
One of the advantages with a battery based solar project is that you already have a backup generator. That is actually worth the money because getting solar with batteries is CHEAPER than getting solar with a backup generator.
Also, in a densely populated area, it’s harder to get a generator because it makes a lot of noise.
Battery backed up solar PV is silent and economical when compared with a backup generation if you were to buy a generator. If you want backup generation and have room for solar, having a battery back up is a great alternative.
The property described here also has solar thermal integrated into it and that can be just as good an option. This is especially true in a condo situation where several owners want to share solar the resource. The physical installation can be easier because shared mechanical infrastructure is common for domestic hot water systems. At S+H we have innovated a fractional ownership model that will allow individual condo owners to monetize incentives. Keep that option on the table.
Q: How do you determine if a client is a right fit?
They have to be committed to solar and have the roof space.
Condominiums are another issues to consider. If you live in a condo, you’ll need to confirm that you have the roof access and roof rights.
Q: If they are want to get a battery based solar system, how do they make sure the process goes smoothly?
There are two huge roadblocks for an installer who has not done this before. First, no having proper documentation of past projects to show to the utility. Second, not having the relationships in place to meet a construction schedule.
I know the exact person to speak with and I have a special letter from the utilities in case we want to do another one of these projects. We have a letter from upper management at a utility that we can use over and over again. If you don’t have this, it will take you a long time to go through the process.