Money saved annual when Solar hot water system installed?

[bernard]

Hi,
I would like to calculate by hand the amount of money saved per one year when one installs Solar Hot Water system, in comparison with conventional electric system. Precondition is that Solar fraction percentage is known in advance (70%).
It does not need to be accurate, just a sort of rule of a thumb.

For example:

Location: Los Angeles
SWH collector area: 4 m2 (43.05 ft2)
Solar fraction = 0.7 (data taken from NREL base case report)
Tilt angle: 34 degrees (location's latitude)
Orientation: south
Average annual solar radiation: 5.96 kWh/m2/day (data taken from http://maps.nrel.gov/prospector)
Energy cost per kWh: 0.20 $/kWh
----------------

daily load = Average annual solar radiation / Solar fraction = 5.96 / 0.7 = 8.52 kWh/m2/day => 8.52 * 4m2 = 34.08 kWh/day

Annual savings = 365 days * 34.08 kWh/day * 0.20 $/kWh = 2488 $

I was wondering if somebody could comment on this.

Is the formula for daily load correct (Average annual solar radiation / Solar fraction) ?
Daily load would be the daily energy demand for particular residential house.

Thank you.

[J.P.M.]

It is necessary to start with how much energy use your daily hot water needs require. That is your daily load. Annual load is the daily load *365 days/year.

The daily load includes heating all the water used at an elevated temp. from its source inlet temp. to its required use temp. Plus any and all associated system thermal losses.

Sizing and cost savings can be complicated - probably beyond the scope of this forum format.

Cut to the chase:

I've got 64 ft^2 of Sunearth thermal collectors next to my PV array in zip 92026. Using estimated expected solar radiation, something called TMY3 data, those (2) collectors should provide about 4,490 kWh/yr. of energy, +/- about 10-20%/yr., depending on the weather. The annual collector thermal efficiency is about 38.8%. That is, over the course of a year, the collectors turn about 40% of the solar energy hitting them into thermal energy that manifests itself as heated H2O. System thermal losses from the tank and piping losses reduce that system efficiency to around 25 % of collected energy being available for use to meet the DHW demand. The system provides about 95% + of the DWH dwelling load with some summertime underutilization by covering part of one collector in the summer to help avoid overheating.

I track that output like a hark and after rebuilding the system in 2008, the output seems a bit better than I estimated, by maybe 5% or so.

Lots of gory details, but that's the bottom line Reader's Digest version.

I like the thermal system I have. However, If I were doing it today, I'd skip the solar thermal, make the PV array bigger and supply the DHW with a heat pump water heater fired with PV supplied electricity from the larger array.

[LucMan]

http://www.solarpathfinder.cOm

This site has evaluation software available.
Excellent software, you can change all the parameters to find your max output settings.