Title: Design a Solar Hot Water Heater System, Part I
Activity: This is a worksheet to size the collector for a Solar Hot Water Heater System. I
have used various parts in Solar Thermal Processes Class and Energy Management
classes. It can be a 1-2 session in-class exercise or a longer term project where students
research background information.
Instructor name: Matt Bullwinkel, SUNY Canton
(Adapted from Florida Solar Energy Center1 and “Solar Hot Water Installers Course”2)
Discipline: Alternative and Renewable Energy
Objective: Students size a collector and tank for a solar hot water heater and estimate
the savings by replacing an electric how water heater by a solar hot water heater. They
employ concepts such as Costs of Electricity, Unit Conversion and Energy Calculations,
Energy Terminology, Solar Energy Systems, Design Trade-offs.
Grade Level: High School/Community College
AET Solar Example Solar Installation in Colorado
(http://www.aetsolar.com/solar_photo_album/)
Florida Solar Energy Center: “Simplified Sizing Procedure for Solar Domestic Hot
Water Systems” FSEC-GP-10-83
(http://www.fsec.ucf.edu/en/consumer/solar_hot_water/homes/index.htm)
2
summer 2007, Sullivan County Comm. College, Loch Sheldrake, NY. Dr. Gay
Canough, Master Trainer, ETM Solar Works, Endicott, NY www.etmsolar.com)
1
1. Introduction:
Solar hot water heaters can be used to provide domestic hot water (DHW) and heat
for home heating. DHW is used for things such as showers, laundry and dishwashing.
DHW is one of the major uses of energy in a home. Often DHW heaters use electric
power to heat the water which is very expensive-however they are very convenient to
install since they do not require the venting of combustion products that natural gas, oil or
propane heaters do.
Solar Energy is FREE and does not have the environmental effects that burning fossil
fuels has. In this worksheet you will design a system and estimate the energy savings that
are possible by installing a solar hot water heater on your home assuming you have an
electric DHW heater. You should complete underlined sections in bold italic print*.
HAVE FUN!
To start:
How many gallons of hot water do you think you use per day? ________gallons/day
How much do think it costs you per year for the hot water you use? $ __________/year
Sun’s Radiation
Collector
Aux.
Electric
Heater
Tank
Figure 1 Solar Hot System
Florida Solar Energy Center
www.fsec.ucf.edu
A diagram of one type of solar hot water system
is shown in Figure 1 below. You can imagine that to
do a complete design of a solar DHW system is fairly
involved. But we can get a good idea of the savings
by making a few approximations.
Find the Collector and Hot Water Tank on the
diagram: we will be determining how big to make
them. A collector or Panel is usually made of tubes
welded to a metal plate covered by a piece of glass.
The plate absorbs the sun’s radiation and heats up.
Water with antifreeze is pumped from the tank
through the absorber tubes, absorbing the heat. The
hot water then flows back to the tank, heating the
water in the tank. Notice there is an auxiliary electric
heater. It is usually only economical rely on solar to
provide from 60-90% of the energy needed and to use
a conventional auxiliary heater such as an electric
heater to provide backup when the sun is not shining.
We will see that this still represents a very significant
savings!
*
Sample data and calculations are shown to illustrate what students would do
The main steps are:
 Establish your Energy Costs:
 Estimate the hot water storage needed (tank size)
 Calculate the amount of Energy needed to heat the water
 Estimate the Annual Cost of Electricity needed to heat the water
 Estimate the amount of Solar Energy that is available at your location
 Calculate the area of solar collector needed
 Determine the savings by installing a solar hot water system
2. Establish your Energy Costs:

Collect the electric bills at your house for the past year1. Tabulate the
Month, usage and bill amount in the table below
Month
Usage
(kWh)
January
February
March
April
May
June
July
August
September
October
November
December
TOTAL
1
Total Bill
($)
2,065
1,783
1,771
1,474
1,009
679
640
1,080
1,026
1,215
1,579
1,443
15,764
$297.66
$280.33
$259.06
$226.74
$154.80
$121.65
$105.27
$175.05
$166.70
$188.85
$233.28
$217.26
$2,426.65
Your instructor can supply you with some sample bills if you don’t have any!

Calculate price of Electricity:
Annual Total Cost / Annual Total Usage = $2,426.65/15,764 kWh= $ 0.15
/kWh
3. Estimate the hot water storage size needed per day

Select the number of people in the family: ____3___
 Calculate the amount of water storage needed: __60_____ gal/day
(Assume 20 gallons per day for first two people, 15 gallons per day for each additional
person. Round to nearest 10 gallons or commonly available tank size)
(Was your beginning guess close?
)
4. Calculate the amount of Energy needed to heat water
Cold Water at temperature TSupply (typically 50oF) is supplied to your house from a
well or city supply. Whatever amount you use from part 3 has to be heated to the
temperature of hot water tank TTank (usually should be set to about 120oF). The formula
for the amount of heat Q required to raise the temperature of water is
Q  mC p TTank  Tsup ply 
m is the mass of water in the tank in lbs. The mass can be calculated by multiplying the
volume of water in gallons times the density of water which is about 8.34 lbs/gallon
m = Volume in tank x Density of water = ___60____gallons x 8.34 lbs/gallon= 501_lbs
Cp – It takes 1 BTU (British Thermal Unit) to raise 1 lb of water by 1o F

Calculate the amount energy needed:
Q= _500_____ lbs x _ BTU/(lb-oF) x (__120__ - __50__) oF =____32,070_ BTU’s/day
How many BTU’s are required annually?
Q= ____32,070_______BTU’s/day x
365 days/year
=__11,700,000 BTU’s/year
It is usually only economical to rely on solar to provide from 60-90% of the energy
needed and to use a conventional auxiliary heater such as an electric heater to provide
backup when the sun is not shining. Typically we design for 70% (see extension #4 at
end though!).
Qdesign=Q __11,700,000 BTU’s/day x 0.7 =__8,190,000 BTUs/year supplied by Solar
5. Calculate the Annual Cost needed to heat the water
using Electricity
First we have to make a conversion between energy units in kWh and BTU’s
3,412 BTU = 1 kWh
So from part our electricity cost is:
___ BTUs/year (part 4) x 1 kWh/ 3,412 BTU x
$/kWh (part 2)
11,700,000_ BTU’s/year x 1 kWh/ 3,412 BTU x _0.15 $/kWh= $ 515 /yr !
(Was your beginning guess close?
Since Solar Energy is FREE we now save the cost of heating the portion that is heated by
solar:
8,190,000_ BTU’s/year x 1 kWh/ 3,412 BTU x _0.15 $/kWh= $ 360 /yr savings!
6. Estimate the amount of Solar Energy that is available
at your location
The amount of solar energy that falls on a collector is called the insolation. One
disadvantage of solar energy is that it is very predict insolation at any given time. You
can imagine that it would depend on the angle of the sun in the sky, the tilt of collector
and the hardest to predict: how cloudy the sky is. A way around this is to use an average
number that is based on many years of collected data for locations around the U.S. This is
shown in the Table 1 on page 8.

Find how much average insolation falls on your location:
Find the city that is closest to your location in Table 1 : __Syracuse NY___
From Table 1, how much insolation falls on per day : __1000_BTU/ft^2-day___
(Use a 45o tilt )
7. Calculate the area of solar collector needed
There are three steps to determine the area needed for the collector of energy that the
solar hot water heater can supply. Solar Collectors have been tested by the SRCC (Solar
Rating and Certification Corporation) to see how much heat they can capture from the
available solar radiation.

Choose a make and model of collector:
There are many manufacturers of solar thermal collectors. You can find one the web. The
SRCC website (http://www.solar-rating.org/ratings/ratings.htm) lists manufacturers. Or
you can use the information from Table 2 below.
Make ___HELIODYNE________
Model ____Gobi 410_______

Find how much solar energy we get for a given area of a selected
collector:
Solar Collectors are only able to collect a portion of the available solar radiation
that falls on them at any given time. Many collectors have been tested. The results are
reported in tables like the one below a listing at the SRCC website
This amount depends on the “Category (Ti-Ta)” (the difference between the inlet
water temperature to the collector and the air temperature) and whether the insolation
conditions are “CLEAR DAY”, “MILDLY CLOUDY” or “CLOUDY DAY”
Table 2 Ratings of Sample Collector
a)Choose category C or D
___D________(Syracuse)
(If you live in a warm climate then choose C, if a cold climate use D.)
b) Choose an insolation category
Your results from step 6, say that on average 1000_BTU/ft^2-day falls on your location
Now Choose an insolation category that is less than or equal to this number:
“CLEAR DAY”
(2000 BTU/ft^2-d)
“MILDLY CLOUDY” (1500 BTU/ft^2-d)
___________
___________
“CLOUDY DAY”
(1000 BTU/ft^2-d)
______X____
c) Find the performance rating for these conditions in Table 2:
For category __D_ and ___“CLOUDY DAY” insolation conditions the collector
provides 4 x 1000 BTU/panel per day = 4,000 BTU/panel per day
(Example : If we have Milwaukee WI Table 1 on page 7 gives us an insolation level of
about 1200 BTU/ft2-d. Milwaukee WI is considered a cold climate so we can use D.
Since 1200 is between the mildly cloudy and cloudy conditions we would use an
insolation category of “CLOUDY DAY” (1000 BTU/ft^2-d). Basically we are saying
that on average the collector will experience an insolation level equivalent to a cloudy
day conditions)
d) Find the number of panels required:
__8,190,000 BTUs/year supp by Solar x 1 year/365 days = 22,438 BTU/panel per day
22,438 BTU/panel per day /4,000 BTU/panel per day = 5.6 panels = 6 panels
(Round up to nearest integer: can’t install a fraction of a panel!)
e) Find the area of panels required:
From table the gross area of this collector is 32.35 (4x8 ft) ft2 per panel
32.35 ft2 per panel x 6 panels = 194 ft2
8. Extensions
Congratulations! You have come up with a preliminary design for a solar hot water
system. Here are some ideas for further investigation:
1) What are there other equipment would you need to design for a complete system?
2) Find the cost of the collector you selected on the web. How long will it take for
your electricity savings to pay for the collector?
3) What adjustments can you make to your calculations to reduce the system cost?
(Hint: Are there any things you can do to reduce your hot water energy cost
WITHOUT installing a Solar system?)
4) If you used Category D and CLOUDY DAY conditions in part 7, your design
would result in a large number of panels which may not be economical. Notice in
Table 1 that the BTU/ft2-day rating triples in moving to MILDLY
CLOUDY=1500 BTU/ft2-day. Table 3 on page 8, shows this is true for about 6-8
months of the year for northern locations. We could design based on this and live
with whatever we get in the winter. How many panels would be required then?
Table 3
Daily Average Insolation by Month
Massena, NY
Month
January
February
March
April
May
June
July
August
September
October
November
December
BTU/ft^2day
530
826.8
1177.66
1479.76
1753.24
1921.78
1928.14
1612.26
1242.32
822.56
490.78
416.58
(http://rredc.nrel.gov/solar/old_data/nsrdb/1961-1990/redbook/mon2/state.html)