Energy conservation first then solar: the sequence for success

May 9, 2010 · Posted in Construction, Lessons Learned from a Green Building Project · Comment 

When we built the house in 2004, solar electricity (photovoltaics) was too expensive for us. Since this was new construction, we were able to focus on the most cost effective measures first. I think we succeeded for the most part, although I’ve learned a lot in the last six years and would do things differently. One thing we did get right was the concept of focusing on conservation first. The building site and orientation capture passive solar heat. Because passive solar is mostly a design issue, there is very little direct cost. Just use your noodle and make the right design decisions. The building shell is tight and well-insulated. Spray foam insulation was spendy, but performs very well and never wears out. It was a good investment given the choices at the time, although it’s one of those things I might do differently. Choosing conservation and passive solar was the obvious choice: a no-brainer.

solar water heating collectors and photovoltaic modules

Solar water heating collectors and PV modules share the roof, but they were installed years apart.

But I wanted to go further, so the next step was some kind of renewable energy system. I had already experienced the joys and savings of solar hot water on a previous house, so I knew I would like it. Plus it’s generally accepted that solar water heating captures more energy for each dollar invested. The actual performance is determined by the local climate (availability of sun) and the equipment selected. In central Oregon, the Sun Earth solar system that we installed is expected to generate 3100 kWh per year of energy in the form of domestic hot water. The performance of solar water heaters varies quite a lot. Performance information is available from the Solar Rating and Certification Corporation (SRCC).

Before incentives, the solar water heater would cost us about $6000. Back in 2004, a photovoltaic system capable of generating the same amount of energy (2.2 kiloWatts) would have cost more than $25,000. So, it made sense to install the solar water heater first and wait for PV prices to come down. Now it was time to think about making the house PV ready. I wanted to do everything possible to prepare the house for a smooth PV installation when the time was right.

The site selection and building orientation that work so well for passive solar also give us a large area of unshaded south facing roof. There’s an old rule of thumb for the optimum angle of a solar collector that says the collector angle should be equal to the latitude. That would be 44°N here. The roof pitch is 6-in-12 or about 21°. Most experts agree that the angle penalty is small, especially because winter sun which benefits from the steeper pitch is not as plentiful anyway. So, my roof is a bit flatter than optimum, but the collectors lay flat and look good.

The solar water heating collectors were positioned off to the side, almost directly above the storage tank in the garage. That left a large open area for PV modules. We relocated a couple of plumbing vents so they would not interfere. Flues, chimneys and surface mounted roof vents would be other things to look for, but we didn’t have any of those. But it’s not just physical obstructions that could foul up a PV installation. Shade is the devil. Even a small amount of shade on a typical PV array can kill production. You certainly don’t want architectural elements, such as dormers or chimneys, sabotaging your PV system down the road.

You also must anticipate the space needed for other system components, including the inverter, meter and disconnect switch. These elements are generally located next to the electric service panel or breaker box. In my case, this all fits within a 3-foot by 3-foot area. One often recommended element is an electrical conduit leading from the electric service panel to the roof. We decided not to spend money on the conduit, because it could easily be run up the side of the house. If a conduit is included in the PV-ready package, you will have to be very explicit about the location of the modules and the wiring.

By 2009, the economics of photovoltaics had changed considerably. State and federal income tax credits along with electric utility incentives were higher than ever. On top of that, the market for PV modules found itself with a supply glut that drove prices to unprecedented lows. This combination of factors brought the ultimate price of a 2 KW PV installation to below $1000. It was time to move from PV ready to PV installed.

Tags: <, , , >