SDHW System Design Notes

There are three steps in the design of a solar DHW system:

  1. determination of solar energy available per unit area of collector,
  2. determination of heating load, and
  3. sizing the collector for cost effectiveness.
  • System sized to provide 90-100% of total hot water use during the warmest and sunniest months of the year. In northern latitudes, where there are large temperature and insolation variations between seasons, the goal is to shoot for ~70% of annual usage so that you don’t overshoot in the summer.
  • Design based on 25 gallons per person per day. Assumes water-efficient, low-flow faucets, shower heads and appliances.
  • Assumes 4×8 panels with 30 sf aperture area. Storage capacity 1.5 – 2 gallons per SF of panel.
  • A Differential Control senses temperature differences between water leaving the collector and the coldest water in the bottom of the storage tank. When the temperature of the water in the collector is hotter than the water in the tank, the differential control operates the circulating pump.
  • A small photovoltaic solar panel option is available to operate the circulation pump, further reducing traditional energy consumption.

UPDATE 7/22/2012

Instead of glazed AETsolar panels, I’m using two of Paul’s Heliocol pool heater panels and a 100 gallon tank-type electric water heater (but used only as an insulated storage tank).  Each HC-50 panel is rated at peak 50,000 Btu/day.  The Heliocol Specs say they are rated for 90 PSI working pressure and 270 PSI burst pressure.  I asked the Heliocol rep about the max recommended panel temperatures and was told 180-190F (they are CPVC).  So I conclude that I can circulate ‘city’ water pressure through them without the need for heat exchangers and extra pumps.  Pay attention to fittings and connections to insure that they are compatible with the temperatures as well.  (CPVC Installation info, FAQ’s)

Calculations: On a design day, two panels should absorb a max of about 100 KBtu.  Let’s say the peak design hour is 1/4 of the daily total, or 25 KBtu.  25,000 Btu / [5 GPM * 8.33 lb/gal * 60 min/hr] = 10 degF TD.  At 5 gpm @ 6′ TDH, (See pump curve below) A Laing D5 Strong/090B PV-Direct Circulating Pump with Check Valve can remove up to 25 KBtu of heat from the panel.  This model has 1/2″ Sweat Connection with Union, Ball, Bleeder and Check Valve. The 710B has 1/2″ FPT fitting, the 720B is sweat.

Assuming an entering water temp of 75F and a final temp of 140F,  a 100 gal tank can store 54KBtu. (Or higher.  At any rate, we need to install a mixing valve on the outlet, and provide a drain to pipe PRV discharge to.) Also wrap the tank with additional insulating blanket.

CPVC Specs, Pressure ratings, dimensions, etc: CPVC-Pipe-(Sch.40and-Sch.80)Condensed

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