Adjusting for Other System Designs

Blalh -- blah

Assumptions Used

These assumptions were used to size the solar

Hot water demand is 48 gal per day which is typical for a family of three
Collector efficiency curve parameters are based on the SRCC efficiency curve for a commercial flat plate collector with a black painted (non-selective) absorber.
Collector tilt is set to the latitude of each city modeled, and the collector faces south.
Cost is based on this cost table for the $2K system with the space heating left out. The cost is adjusted up or down depending on the collector and tank size used.
The existing water heating (before solar) is assumed to be electric at 12 cents per KWH.

See the description of the $2K System for the details on the collector, tank and controlls assumed for this system.

The tank size is done at 2.5 gallons per sqft of collector, but is not allowed to go below 100 gallons in order to have room for the pipe coil heat exchanger. It could be argued that the tank capacity per sqft of collector should depend on climate (larger for warmer climates) -- I looked at this a bit and it did not make much difference on DIY system cost or savings or performance.

Hot Water Demand Not Equal to 48 Gallons Per Day

If your hot water demand is more than or less than the 48 gallons per day that used, you can just scale up (or down) the collector area, energy saved, cost of energy, and CO2 emissions by the ration of your hot water demand to 48 gallons.

For example, a system in Denver that delivers an 80% solar fraction with a hot water demand of 48 gallons per day requires about 40 sqft of collector area, saves about 2.3 mega watt hours a year, saves about $275 a year in energy costs, and saves about 2.6 tonnes a year in CO2 with a payback period of 2.9 years. All of these read of the graphs on this page...

If instead, the system had to produce 60 gallons of hot water a day (25% more than 48 gallons), then all of the numbers above would be 1.25 times greater, so,..

Collector area goes from 40 sqft to 50 sqft

Energy saved would go from 2.3 to 2.9 mega watt-hours per year

Cost savings would go from $275 to $344 per year year

Carbon emissions reduction would go from 2.5 to 3.1 tonnes per year

There would also be a change in cost of the system. Using the cost formula formula (next section), the system that produces 48 gallons of hot water a day has a Total Cost = (40 sqft)(8.5$/sqft) + (2.5*40 gallons)($1.8) + $426 = $946

The system that heats 60 gallons of hot water per day has a Total Cost = (50 sqft)(8.5$/sqft) + (2.5*50gallons)($1.8) + $426 = $1076

The payback period would improve a little to ($1076/$344) =

Cost of the System

The cost of the system is calculated as:

Total Cost = (Collector Area sqft)($8.5) + (Tank Cap gals)($1.8) + $426

So, a system with 60 sqft of collector and a tank size of (60 sqft)(2.5 gals/sqft) =

Where the $426 covers cost of plumbing, pump, potable water heat exchanger, controller, ...

All of these costs are for materials only with you supplying the labor.

If you have more accurate cost data for your system, just use your data.

Collector Efficiency and Orientation

I assumed the efficiency for a glazed collector with a black painted absorber, and that the collector has a tilt equal to the local latitude, and that the collector is facing due south.

If any of these are not true for your case, the easiest way to correct would be to do your own RETScreen runs and put in the correct value for efficiency, tilt, and azimuth. Details on how do this here...