Calculations for Solar Hot Water Systems


In order to design an effective hot water heating system for your solar water heating system, you will need to determine the size and equipment and predict the energy consumption needs. Software programs can be obtained to calculate peak loads and heating efficiency. Software costs money and many people want to know how to perform this analysis without software tools.

Several factors will impact performance. The amount of sun available is one factor; the outdoor temperature is another major factor. Here is a simple method of calculations your needs.

The first step is to estimate daily hot water needs in gallons (Gallons) and a nominal tank size (Tank Size). For two people, the average needs are 40 to 50 gallons per day. Add 15 to 20 gallons for each additional person or additional bedroom in the home to allow for family expansion. Tank size should be large enough to hold slightly more than the needs for one day. If you determine that 70 gallons per day should provide for your need, select a 75 gallon tank.

GALLONS _________________

TANK SIZE ________________

Next, determine the temperature of the cold water that will supply your solar water heating collector panel (ColdTemp). To do this, collect water and use a thermometer to find the temperature on several different occasions. Average the figures to create ColdTemp data.

ColdTemp _______________

Determine the energy needed (BTUNeed) to raise the water from ColdTemp to desired temperature. 122 degrees F is average for desired hot water temperature. If you need higher or desire lower temperature, determine the temperature by using a thermometer to check the temperature of hot water that you want.

To calculate BTUNeed, use the following formula:

BTUNeed = 8.34 X Gallons X (122 – Coldtemp) X Standby loss factor

If you do not use 122 for your desired water temperature, change the 122 in the formula above to your own desired hot water temperature.

To add the standby loss factor, determine the insulation on the storage tank you will use. If you have 1 inch foam or 2.5 inch fiberglass insulation, use 1.20 for the standby loss factor. If you have 2 inch foam insulation, use a factor of 1.12.

Next, you will need to determine the size of solar collector panel you wish to build or purchase. First, determine the penalty factors that will impact sizing. From the chart below, list your system factor.

System Factor Chart

Direct system with no heat exchanger 1.20

Indirect system with heat exchanger 1.30 between collector and storage tank

Systems with SRCC system certification 1.00 and Q NET ratings

To determine tilt factor, you'll need to determine the pitch of your roof. Most common are 20 to 30 degree pitch which has a tilt factor or 1.0. A flat roof carries a tilt factor of 1.25, a 3 to 7 degree pitch carries a factor of 1.15 while 7 to 12 degrees of pitch carries a factor of 1.09, 12 to 16 degrees carries a factor of 1.5. A roof pitch of 30 to 37 degrees carries a factor of 1.01, 37 to 43 degrees has a factor of 1.04. A 43 to 50 degree roof pitch carries a factor of 1.12.

If you wish to determine an extremely exact tilt factor, you'll want a site analysis performed by a solar energy professional. These numbers are meant to give you an accurate estimate for do-it-yourself calculations.

Tilt Factor ________________

Orientation factor is determined by which way your solar collector will face. A south or almost directly south facing collector carries a factor of 1.0. Southeast or southwest orientation has a factor of 1.15, east or west facing collectors have an orientation factor of 1.4.

Calculate your penalty factory using this data and the formula:

Penalty = System factor X Tilt Factor X Orientation Factor

Next think about how much of your hot water needs you wish to have met by your solar hot water system. This will be the RateRequired. The national average is 70%, but you can use a higher or lower number if you choose.

RateRequired = BTUNeed X .70 (or percent your determine) X Penalty

The RateRequired figure will equal the BTU/day you will need.

Next you need to determine what the efficiently of the solar collector you wish to use will provide. This is very easy for a purchased solar collector because there will be an FSEC label stating the BTU Rating which is the BTU per day the collector is capable of delivering. If you plan to build your own collector, locate similar collectors and use the ratings from that panel.

BTURating _______________ is BTU per day from label or similar collector label

Gross Area = ___________________ feet squared.

To determine how many solar collectors with the specifications you entered above, calculate;

Number = RateRequired/BTURating rounded to the nearest whole number.

To learn the total area of your collectors:

TotalArea = Number of Collectors X Gross Area

TotalArea = ___________ feet squared

Now, calculate the solar fraction:

SolarFraction = % of need met which is average of .70 X Number of collectors/number

SolarFraction = _______________

If the solar fraction is less than .65, your system is probably undersized and you should consider adding another collector or using more efficient collectors.

This information will help you ensure that solar water heating system you are considering will meet most of your needs. If you plan to have backup hot water heating capability, 70% will represent a huge savings. Only in the most temperate climates could a 100% capability be expected during the coldest part of winter.

About the Author: Anita van Wyk strives to make make more people aware of the benefits of using solar energy through her website

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