[RE-wrenches] PV Estimate for Vaporware Building

[Jeff Yago]

Luke,

I will give you 2 answers and they are both right, depending on circumstances.

1. In the back of my now out of print book, I had a simple fill-in-the-blank form to size solar pv systems. The first number you enter is how much money the client was willing to spend, then it works backwards to calculate what utility savings a system will produce per year based on the size of system that amount of money will buy.  We finally went this route because we would spend days and days sizing solar systems based clients saying something like "I want to off-set "X" percent of my monthly electric bill."  Of course this requires lots of data and calculations to produce this answer, then we would present the final system design to do exactly what the client asked for and the first thing they would say was "But we only can spend "X" dollars" which was usually about one-tenth what that size system would actually cost.

2. All LEEDS and almost every commercial and institutional building project requires a mechanical engineer to design the HVAC systems. 30 years ago this was done by hand using fill-in-the-blank heating and cooling zone load calculations. Then each calculated zone load sheet was grouped into each separate HVAC system so the duct sizes, heating and cooling coil sizing, and the central air handling units could then be sized.  Finally these we added together to size the central boilers and chillers.

Today all of this is done using computer software and every HVAC system engineer has this software. The most common are from Trane and Carrier, but there are other systems including add-ons that match to an Auto-Cad drawing file and uses the drawings showing the ductwork to calculate the duct sizes and bill of material lists without the engineer having to enter any actual data.  Some can even calculate the exterior wall and window areas directly from the CAD drawings and use this as inout data for these HVAC loads programs.

Every building design funded by a state or federal government now also requires the HVAC engineer to calculate the projected energy usage for the building so the owners can consider which type of heating and cooling source results in the lowest operating costs.

Although we have been using energy analysis software for years like DOE-2 that can do this energy analysis as stand-alone, they require reams of data input, most the same as you would need to size the HVAC systems. For example, you would need to enter the area of each wall, window, door, roof, floor, and their compass orientation. For each device using electricity, you enter data for its operation. Got an elevator, you would need to enter its motor HP and how many hours it runs each day. These programs even allow you to enter the local utility rates so it can calculate the kW demand charges as well as kWh usage.  Anyway, since much of this data (i.e. wall areas) have to be entered to size the HVAC systems, now most of these major HVAC programs can also produce this utility usage data. Since these programs use weather "tapes" for each specific geographic area, which has the temperature, humidity, sunshine, and other weather data for each 8,760 hour of the year for each location. This means if you do a good job of entering this data, you can get a fairly accurate estimate of future energy usage based on the occupancy hours, space temperatures, and outside weather conditions.

Contact the engineers who are designing and sizing the HVAC systems for this project and they will either be able to answer your energy usage questions, or they can give you a CD of all input data which can then be entered into your own software and do this yourself. However, don't be surprised if you have to go back to step #1 above!

Jeff Yago
DTI Solar

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