**Solar Heat
in December
**

by

John
Canivan

Radiant energy from sunlight striking the North East USA mainland during
the month of December is minimal. To be objective about data collection I use
this experimental, MTD sun shed, isolated from conventional sources of heat. The
array of MTD collectors have a combined surface area of about 8 m^{2 }for
collecting heat and a 200 gallon drum storage system for storing the heat.

Unfortunately the shed is surrounded by trees and oriented 20 degrees
East of due South so that an optimum daily heat harvest is impossible.
Fortunately there is no shading between the hours of

According to the U.S. Solar Radiation Resource Maps the energy available
to m^{2} on

From the above graph observe the intense radiation available on December
22 and December 23. On these days the radiant energy peaked at 650 W/m^{2}.
On December 14, 15 and 18 scattered sunlight was available for brief intervals
but a total of less than 6 hours of direct sunlight was available during this 9
day interval. Of course the amount of radiation will vary from year to year and
from day to day, but it appears the average, usable, daily energy availability
during the 2008 December on **1KW/m2/day. **This
is just a crude estimate based on the limited data.

**
**The December heat harvest from six MTD collectors will be examined
later but first let’s take a peak at the power harvest from a .5m^{2}
PV panel during this same 9 day interval. This PV power output may be calculated
by observing the voltage across a 5 ohm load placed across the terminals of the
PV panel. Notice the similarity between the flux intensity graph and the voltage
output graph of the PV panel.

On December 23 we observe the peak voltage output from the
PV panel is about 10 Volts. This translates to a power output of 20 watts. Since
the sunlight intensity is only about half the full sunlight intensity we can
estimate that the full intensity output from the .5m^{2} panel would be
close to 40 Watts. This is about right since max PV output is rated at 50 Watts.

**Heat Gain from the MTD
Sh ed**

We have just observed sunlight intensity and PV power output over a 9 day interval in December. Now we’ll compare the heat gain of the sunshed with the average daily radiant energy available in an ideal situation. We'll use 1kW/m2/day for a reference point and .4 as an adjustment factor to deal with the sunshed orientation and shadows. 1kW = 3000 BTUs.

Since the sunshed glazing has a surface area of 8m^{2} we would
expect the heat energy harvest over the month of December to be in the
neighborhood of:

SOLAR ENERGY AVAILABLE = 8 x 3000 BTU x30 x.4
= 288,000 BTUs

The solar activity on and in the sunshed from December 1-23 appears confusing at first. This data has been collected at four minute intervals from four temperature probes attached to the: collector array, the last storage drum, the middle storage drum (average storage temp.) and the ambient temperature. As you can see the ambient temperature fluctuates from 12 F to 60 F. the collector temperature fluctuates from 12 F to 110 F and the average storage temperature fluctuates from 100 F to 50 F

^{2} MTD
array. On average a drop of 4^{0} F/ day is observed when the pump is
off and gross average temperature drop of only 1^{0 }F/day. The solar
heat gain may be estimated from these temperature differences.

^{0}/day

temp drop with solar
= 1^{0}/day

**Q = solar heat gain
for month of December **(4-1) x 200 gal x 8 x30 day = 144,000 BTU

=

**Efficiency of MTD
array **144,000 BTU/288,000 BTU
=50%

This method of measuring collector efficiency is done by measuring temperature changes in the middle storage drum. This warm drum represents the average storage temperature of the 200 gallon system.

144,000 BTU’s of heat may be insufficient to heat a house, but this method of collecting and storing heat demonstrates the potential to collect and trap the sun’s limited heat energy with a multi drum system in the month of December. If these heat storage drums were allowed to release their heat to a home or a DHW system a net heat gain would be observed even during this dark month. The above graphs demonstrate the quantity and quality of radiant energy available for the entire month of December. A daily analysis may also be performed.

Solar flux density peaked on Dec. 22 &23 between (9AM and noon to turn on the pump and harvest heat energy, but sunlight intensity on Dec, 20 &22 was too low for heat gain.

**HEAT LOSS FROM 200
gal. STORAGE
**Storage temperature Dec. 20th = 62

Storage temperature Dec. 22th = 54

That’s a 4 F drop per day. Notice the increase in ambient
temperature Dec. 21 has no effect on storage temperature.^{
}

**HEAT GAIN TO 200
gal. STORAGE (****9AM**** TO ****noon****)
**Storage temperature Dec. 22th

Storage temperature Dec. 22th

This is a gross storage temperature rise of 5

**SOLAR HEAT GAIN PER/hr
HOUR** **=** 5 x 200 x 8 = 8,000 BTU

**SOLAR ENERGY
AVAILABLE/hr = **8 x 3,000 BTU x ( flux density factor .65)
= 15,600 BTU

**MTD
COLLECTOR EFFICIENCY**
= 8,000/15,600 = 51%

**INPUT/OUTPUT ** method of
measuring collector efficiency

The pump turns on when collector temperature is 20^{0}
F above storage temp and shuts off when collector temp is within 2^{0} F
of storage temp. Without water flowing collector temp could reach 200^{0}
F in mid winter with an ambient temperature of 20^{0} F. In mid summer,
when sunlight is most intense, MTD stagnation
temperature climb higher. Richard Heiliger from Richmond Utah recorded a
stagnation temperature of 265 F in July 2008 inside his MTD collectors, and his collectors
are still alive and well. Of course when the
pump is on and water is flowing through the TDM we would expect the collector
temperature to drop within 20 F of the storage temperature as heat is transferred
to storage containers. By knowing the flow rate of the pump and the difference
in temperature between the water entering and leaving the collector and by
knowing the sunlight intensity we may also estimate collector efficiency.

From the solar flux graph above we know that that a peak sunlight intensity of 650 Wm2 was reached at 10 AM on December 22. Converting this into heat energy per square meter we get 3000 x .65 or about 2000 BTU/m2/hr available.

**Input
and Output temperature of MTD Collector
**

**
**

Observe the collector input and output difference is 12^{0}
F at 10 AM when solar flux peaks. Since the flow rate of water through the array
is 2 gal/min and since 2 gallons of water has a weight of 16 pounds we can
estimate the heat gain/min.

Q/min. = 12^{0} F x 16 lbs. = 192 BTU/min.

Q/hr =
192 x60 = 11,520 BTU/hr

**AVAILABLE ENERGY**
= 2,000BTU/m2/hr x 8m2 =
16,000 BTU/hr

COLLECTOR EFFICIENCY
=
11,520/16,000 = 72%

**COLLECTOR EFFICIENCY
SUMMARY
**

* 1. Input/Output heat gain method
72%

2. Daily heat gain method
51%

2. Monthly heat gain method
50%

* The solar flux energy available estimate of 2,000 Watts/ m2 for this Input/Output estimate was made using a home made pyrometer calibrated from Richard's and Gary's pyranometer so this estimate may be off a little. There are many factors that go into calibrating collector efficiency such as flux intensity, temperature differential and flow rate but I believe all my estimates are within the ball park and the true MTD collector efficiency is somewhere between 50% and 70%.

**
**
The month of December may not be the best month for solar energy but it
provides an interesting worst case challenge. Since both the MTD array and the
PV panel are pitched at 45 degrees I’m confident the heat and power harvest
from the experimental sunshed will improve as the season unfolds. I can only hope
that home builders and home buyers will soon recognize the
possibilities of energy efficiency and solar alternatives. Minimal back up power and heating systems
are a reasonable precaution, but the sun will
always be with us so let’s use all we can.
An ideal, energy efficient solar home requires thinking and planning. So
let’s think before we plan and plan before we build. Retrofits are still
possible if you have a roof oriented in the right direction. See what Richard
did with his roof. http://www.builditsolar.com/Experimental/MTD/MTD.htm