Wind Load On Solar Collector Excel Spreadsheet

Investing in a solar domestic hot water (SDHW) system is a smart solar solution for most homeowners. This proven and reliable technology offers long-term performance with low maintenance. And with federal, state, and utility incentives available, these systems offer a quick payback—in some cases, only four to eight years.

A thoughtfully designed SDHW system could provide all, or at least a significant amount, of your household hot water needs for some portion of the year. The California Energy Commission estimates that installing an SDHW system in a typical household using electric water heating can shave 60 to 70 percent off water heating costs. To get the most for your money, you'll want a properly sized system that offers the best performance in your climate. Here's what you need to know to size right—before you buy.

Efficiency First, Then Loads

Before you install a solar hot water system, insulate pipes and storage tanks, install high-quality, restricted- or low-flow faucets and showerheads, and lower your water heater's thermostat setting if possible. Making these improvements and repairing plumbing leaks will minimize losses, reduce your hot water demand, and make your solar hot water system both smaller and less expensive.

To size your solar hot water system, start by estimating your household's hot water use—its loads. Calculating a household's actual hot water load can be a social science exercise (people have very different water use habits), but a

SDHW Advantages

• Year-round usage

• Relatively low installed cost (often less than $5,000)

• Federal and state rebates or tax credits available (see Access)

• Small area required for collector mounting (usually less than 80 sq. ft., and sometimes less than 40 sq. ft.)

• High efficiency—more than three times that of PV systems

• Integrates with existing plumbing systems

• Variety of system types to meet specific needs

• Proven technology and system designs

• Quality components widely available

• Expandable (with proper design)

• Long system life

• Low maintenance

• Large energy displacement

• Significant utility bill savings possible

Shower Power

After bathing, showering generally is the highest hot water consumer. If you really want to get a handle on your hot water use, you can measure the flow rate of your showerhead with a bucket and a timer, in gallons per minute (gpm). You will also need to measure the temperature of your shower water (fill a cup while you're showering) and that of the cold water supply (preferably while you're not).

Measuring the water temperature and timing the length of your showers will allow you to calculate energy consumed. For example, a 10-minute shower at 110°F (heated from a 50°F supply) with a flow rate of 1.5 gpm (a high-quality, low-flow showerhead) would result in an energy consumption of roughly 7,500 Btu.

Volume (gallons) x Temperature rise (°F) x 8.33 (the density of water multiplied by its specific heat) = Energy (Btu)

Even though the energy use is relatively low, it must be supplied at a high rate (power). That rate of energy usage (45,000 Btu per hour) would have to be matched by the input of an on-demand water heater, but with sufficient storage, we can do the job with low-power solar collection.

Another helpful conversion factor is: 1 watt equals 3.412 Btu per hour. With this, you can convert your shower power from 45,000 Btu per hour to 13,190 watts, which would require 55 amps from a 240-volt electric element—that's what an on-demand electric water heater would have to supply to keep you in hot water!

Load & Collector Sizing Calculations

Some up-front number-crunching can help you size your SDHW system appropriately for your household, saving you money from the get-go. The example below sizes a system for an efficient household in Des Moines, Iowa, that uses about 60 gallons of hot water daily.

1. Calculate your daily household water heating load. Two formulas are particularly important to calculating hot water loads:

Volume (gallons) x Temperature rise (°F) x 8.33 (the density of water multiplied by its specific heat) = Energy (Btu)

Suppose you heat 1 gallon of water from 50°F to 130°F. The temperature rise is 80°F, so the energy formula would tell you that 666 Btu (1 x 80 x 8.33) are required. A family of four, using 15 gallons of hot water each, would require:

60 gal. x 80 (temperature rise) x 8.33 (lbs./gal.) = 39,984 Btu

2. Determine your site's average daily insolation and equivalent SRCC "Sky Type." Use the PVWatts online calculator, or an equivalent source (see Access), and convert the KWH/m2/day figure to Btu/ft.2/day, using the following:

Des Moines receives an average of 4.83 KWH/m2/day (collector at a 41.5 degree tilt angle). Applying the conversion factor:

4.83 KWH/m2/day x 317.1 Btu/ft.2/day = 1,531.6 Btu/ft.2/day

This available solar resource most closely matches the SRCC's "Mildly Cloudy" (1,500 Btu/ft.2 per day) sky-type category.

3. Categorize your climate. For all but the coldest locations in the United States, using the "C" category will give you a reasonable estimate. (For more tips on improving your estimate's accuracy, see the SRCC Collector Ratings sidebar.)

4. Obtain collector performance output data from the SRCC Web site (see Access). One popular 4- by 8-foot, flat-plate collector produces about 24,000 Btu per day in Category C and "mildly cloudy" conditions. If we assume system losses of about 20 percent (80 percent efficiency), one collector can be expected to produce about 19,200 Btu (24,000 Btu x .80) per day—almost half of the family's hot water needs.

To take advantage of the federal tax credits, this family would need to install two collectors and have at least 64 gallons of storage capacity, which would provide most of their annual hot water requirements.

You can find an Excel spreadsheet to help you through the process of estimating a collector's output at www.homepower. com/promisedfiles. SRCC also provides comparisons and ratings of prepackaged SDHW systems. See their Web site ( for details.

Example Collector Data*

Thousands of Btu/Sq. Ft./Day Mildly

Clear Cloudy Cloudy

Category (2,000 Btu/ (1,500 Btu/ (1,000 Btu/ (T—Ta) ft.2 per day) ft.2 per day) ft.2 per day)

A (-9°F)

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