The 1909 Franwall Solar Story

• The most cost effective recommendation was upgrading attic insulation, including sealing the attic. An action well subsidized by Pepco. This made sense, so I used their recommended installer [before: 1 2 3 , during, after 1 2 3 ].
  It turned out that in order to receive the Pepco rebate, proper exhaust from my existing gas boiler was required. And it failed, which means no rebate (the idea is that with a well sealed house, fumes might build up. Not a problem with a leaky house). After a cheap/lame attempt to fix the problem (switching the vent from boiler to chimmey from 3" to 4"), I ended up installing a steel liner in my chimney (actually, the part of the chimney dedicated to the gas boiler exhaust). Which pretty much ate up the rebate. Though now I have an up to code house ready for a high-efficiency boiler.
• I also installed high-efficiency AC/HP and fan coil (so not as much need for a high-efficiency boiler). Redid some duct work too (might not of needed to, and insisted on all hard duct, which turns out to be a bit noisy. Oy)

Moreover the company recommended removal of a few trees next to the house. Since trees-next-to-house isn't such a great thing anyways, I had it done: removing trees on west side (before and after) , trimming back a tree on the northeast corner (so as branches would not overhang back of house), and topping 15' or so off of large pine trees out front (to reduce shade and limit damage if they were to fall in a storm). We left a few bird-habitat branches!

I also replaced part of the roof. The flat roof was 20 years old, nearing the end of its useful life. The sloped roof was about 15 years old, so had another 10 or more. Rather than replacing all of the roof, I just replaced the flat part over the new addition (before and after). That also allowed me to add insulation (that could not be added by the insulation people, since they could not get their blowers into the small attic space above the flat roof).

• flat roof
• east-facing roof
• north-facing roof

Roughly speaking, there are two ways of accomplishing this inverting.

1. Central inverters. This is an older technology. DC power from panels is fed into a central inverter, typically located in the basement near the circuit breaker box.
2. Micro-inverters. A relatively new technology. Each panel has a small inverter attached to it, that converts DC to AC and then sends the power to your circuit breaker box (I am abstracting from safety switches and combiner boxes...).

However, central inverter PV installations have a design issue. For several reasons (such as increasing voltage to reduce line losses), PV panesl are usually serially wired in strings. This makes them vulnerable to shade -- since the output of all panels in a string can only be as great as the worst (i.e.; most shaded) panel in the string! That means a branch partially shading just one of a dozen panels can cut the output of the whole array of panels by a lot!

Micro-inverter avoid this problem, since each panel is independent. There are ways of limiting this problem with central inverters, by having several inputs (so that each string is directly wired to the inverter), or by adding DC optimizers (at about a 20% cost increase per panel).

With an odd installation -- shade, or orientation at different angles -- these kind of problems are more important.

What to do? One could just waste the power, but that would be kind of nuts. If one was off-grid, battery backup is used. But batteries are expensive (often greater than the cost of the panels).

Instead, most PV installs in urban settings use a grid-tie architecture. Basically, PV power is fed directly to the main circuit breaker box. From there, it either goes to loads in the house (lighting, AC, etc) or if more power is being produced than needed -- it backfeeds to the grid. This means your meter spins backwards -- your house becomes an electricity producer, not a consumer.

In a sense, the grid acts as an infinite battery, able to take whatever excess power you may produce.

In most states, the utility is obligated to accept PV power produced by households, and to credit this power to your electrical bill. In other words, they pay retail for the power you (the PV equipped home) provide to them. The utiltities don't like this very much, they would much rather buyit from you at wholesale!

Actually, this grid tie architecture does complicate the utitlity's power management regime. The utilities have to be adjust production to demand across the grid. They are used to doing that with their centrallized generating facilities. Adding a whole bunch of small distributed facilities (homes with PV panels) makes the balancing more difficult. So long as the fraction of power supplied by homes with PV panels is small, it isn't a huge problem. But should this fraction grow, it could become problemmatic (i.e.; how to adjust centrallized generator output as clouds move in front of the sun).

Therefore, grid tie systems must have a safety feature: during power outages, no power is fed from PV panels to the grid. The simplest way to do that is to simply shut down PV production.

What does that mean? During the times when PV power would be very useful -- during outages -- it is not available! That seems wrong, but it is an inevitable result of using the grid as a battery. And since 99% of the time the grid is working, that's a tradeoff that can make sense.

Still, it does seem just wrong. It seems sensible to consider ways to finesse this problem, ways of building a hybrid system that uses both your own batteries and a grid-tie connection to the utility.

Types of hybrid systems -- systems with both battery backup and grid-tie.

The basic idea is to seperate the PV power from the main circuit breaker box. PV power is fed into an intermediate device which can use the PV power in 3 ways: charge batteries, feed to a sub-panel, or send to main breaker box (and hence to the grid). When an outage occurs, this intermediate device can quickly switch to battery power (it "inverts" DC power from the batteries to AC power). This battery provided power is sent to a sub panel that supplies the most important loads; such as refrigerator, heating system, and basic lighting. This sub panel is isolated from the main panel, hence from the grid. Furthermore, the PV panels "see" the AC power from this intermediate device and will continue to produce electricity.

This means two additional components are needed. First, an intermediate device -- such as an off-grid inverter. Secondly, batteries.

Batteries are not cheap. A good quality 200 AH 12V battery costs $600. Four of these yields about 9.6kwh of power. Actually, these are usually only discharged to 50% -- which means something like 5-6kwh of power. If you are frugal, that's about 6 hours of power.

Details galore: I ended up choosing an AC coupled system using Magnum off-grid inverter and about 11kwh of batteries. This was minimal: I anticipate using my generator to charges these batteries at night. Still, if it works this setup provides UPS protection for much of the house, protection against short outages, and a great reduction in generator run time.

• Articles describing AC coupling: Solar Pro, Renewable Energy World, or a Magnum white paper,
• A table listing some options
• Thoughts on design choices (July)
• Thoughts on design choices (August)
• A slide show diagramming some possible layouts
• Thoughts on design choices (August)
• A slide show diagramming actual possibilities (based on some of the equipment I finally chose).

Kenergy ordered a enclosure.and when it arrived it was... pathetic. And too small. After expressing displeasure, I decided that I would have to bite the bullet and go over budget -- I would offer to pay for most of a good battery enclosure. That meant about $1000 for a solid steel construction from Midnight Sun. After some convolutions, where I paid for it even though Kenergy ordered it, the order was placed in early March.

And I waited until middle of May for delivery. Seems like this was a proposed product from Midnite Solar, rather than something that was regular inventory -- they hadn't gotten the bugs ironed out. Indeed, when I finally got it, a hinge was missing from the door. It took 2 weeks, and some embarrassment by the production line manager, for that to get mailed. But it is a very sturdy enclosure!

While waiting for the enclosure, Kenergy did nothing. Even though there were thing to be done, such as figuring out why an entire string was not working. And by April, I was just tired of it. So I gave them a 4 week deadline, wherein I sent 3 registered letters. With no response.

It seems obvious that they were no longer interested in finishing the job. Fine by me.

Of course, my sympathies are limited: these challenges weren't that big, only one of them was a requirement, and I am paying more than average.