By David Watts

If you drive past the Parking garage at the Ajax GO station, or the Erindale GO station, there is nothing visible that sets them apart from the other dozen or so parking facilities Metrolinx has constructed over the past 2 years.

You must get to the top floor to see the difference – approximately 1,200 solar modules on each of the those roofs, and a purpose built inverter room, designed specifically for the unit that converts  the direct current generated by solar modules to the alternating current that powers our houses.

Each garage will generate approximately 342,000 kWH per year, which is the equivalent consumption of 32 or 33 average Ontario homes.

It can also be looked at as offsetting the consumption of the parking facility.

Consumption projections at Erindale showed projected electrical consumption of about 1,000,000 kWH per year. The solar on the roof covers about a third of the annual needs, but if you break it down by season, it covers the summer consumption almost entirely. Winter is another story- the days are shorter and cloudier, and the nights long.

Long term analysis will show us if actual consumption reaches the projected levels, as all the lighting is LED, and lights are on timers and photo-sensors.

How do you plan a solar garage?

It is actually quite simple.

The racking system which connects the solar modules to the building is an engineered, ballasted system.  Wind tunnel testing shows us just where we need to place the weight to ensure that the entire array stays put, no matter what nature throws at us.

Typically the universal load for this ranges from 3 PSF to 6 PSF. If the parking facility in the design stage has a top floor that has a minimum extra 7 PSF allowance designed into it, I would call that solar ready. 10 PSF would make it a solar rock star, ready for anything.

The facilities at Ajax and Erindale utilize central inverters. These are 250kw units, ten feet long, six feet high and about 7,000 pounds.

Metrolinx had rooms built specifically for these inverters, which were hoisted to the roof at the conclusion of the pour, and before the rooms were finished.  Our current design strategy has shifted since these installations.

Today, we would design a 250 kW project using twenty-five 10 kW inverters, twelve 20 kW units and one 10 kW, or eleven 22.7 kW inverters. This means you do not need to include the purpose built room in your design. The smaller string inverters are mounted directly on the roof, as part of the array, the smaller size make roof loading a non-issue, hoisting is not required, and the distributed nature means if one unit is offline, you only lose minimum four percent of production, versus 100% if a central unit goes down. The inverters are combined at an AC re-combiner. All the inverters on the roof feed this single panel. From there all the electricity is now in a single run.

The electricity must get from the roof to the meter room, and this is where close communication between the architects, the general contractor and the solar designers come in.

When the conduit is being run, the solar array will need to go in at the same time. Typically the run could be as large a three runs of 500 MCM, plus neutral and ground. It is best if the solar team supplies stamped electrical IFCs for review before this stage, so that everyone understands the scope of conduit work that needs to be done.

Now that the electricity is in the meter room, there are two ways of connecting to the grid- net metering, and the feed in tariff. Typically feed in tariff projects connect line side, and net metered projects connect load side, behind the meter. Each has a connection point as defined by connection rules with the local utility. Involve them early, and keep them up to date on progress.

When the type of connection is settled, and the switch gear is being ordered, it is helpful to have an extra set of lugs at the point of connection so the solar electricians don’t have to drill their
own holes.

In short, a parking garage is solar ready if it has an allowance of ten PSF on the roof, and there are extra lugs in the switch gear. That is of course the bare minimum and the more dialogue you have with the solar team at the design stage, the more the finished project will be integrated into  the mechanics of the finished facility, rather than as an after-market addition. ν

David Watts, Technical Director, Solera Sustainable Energies Office
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