The Story of RHP4
This project was a small addition, could we heat the addition and provide domestic hot water too? "Sure" we said. One interesting decision: what source for the backup heat? The customer had an existing gas water heater. Usually, in solar, you want to minimize standby losses - losses from hot water through the insulation as it is just sitting there, waiting for you need hot water (standby losses are higher than typical because you store the energy by keeping the tank hotter than it would be otherwise). And a gas water heater has a flue (vent) - that merrily carries away heat 24/7! That argues for an electric tank. Our customer mentioned wanting to add a photovoltaic (solar electricity) system in the future, which made the electric backup decision even easier! This design decision ended up causing us some grief. In our market, electricity costs 3 times as much as natural gas! So for six months of the year, 3 evacuated tube panels are oversized (by about one panel) for a family solar water heater. It really doesn't matter what the backup is. For 3 months, you are using your backup on and off Not a big deal. But for 3 months of the year (winter) you are REALLY into the backup as you have your biggest load -space heating that small addition- plus your least amount of sunshine. So our customer got rather notable electricity bills and asked us "wasn't solar going to reduce our energy bills?" We eventually solved this one by changing the control strategy so that the backup for the space heating was the gas water heater. And we work a lot harder now at expectations management (we didn't communicate clearly what the system could and could not do).
System at a glance
System type: Space, DHW Collector type: Evacuated Number of panels: 3 Freeze protection: Drainback Predicted annual BTUs: 31,617MBTU Actual annual BTUs: 30,669MBTU Annual heat load: 44,085MBTU Solar Fraction: 70% Fossil fuel: electricity System costafter rebates: $12,046 Payback: 9 years
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