MTD Data
This experimental MTD shed in my Long Island back yard is being used to gather data throughout the winter. it's framed with 2x3's and sheathed with inexpensive Luan. Luan as you know is not waterproof BUT if you give it a few coats of KILTZ it will hold up to the elements. I used solid insulation scraps on the inside so the R factor for the 8 x12 shed is about 1 if you take into account air infiltration and other factors. Inside there are six 20 gallon plastic storage tanks hooked in series. They too are insulated with solid insulation. Tank insulation is about R=3 so heat retention is not great BUT it has been enough to keep the tanks from freezing so far. As a matter of fact the lowest recorded temp of the tanks this year has been 50 F. A 50 watt PV panel drives a $30 RULE bilge pump when the differential thermostat kicks in. All together the six collectors, shed, tanks, and pump cost under $1000. The PV panel ( the only source of power) is on loan from Farmingdale University. It would retail for between $200 and $300.
As you know solar heat collection is a function of many factors such as
1. surface area for heat collection (about
7 square meters for these 6 collectors)
2. ambient temperature ( low temperatures diminish heat collection a bit)
3. solar flux... very important ( a clear bright sky makes up for low
temperatures
4. orientation ( due south is best but not always possible or practical.
I'm off by 20 degrees.
5. inclination ( subtract about 1% efficiency for every degree off of perpendicular
radiation. I'm optimized for
February 10 at a pitch of 50 degrees.
6. insulation. Most of your heat will be lost through the glazing so don't go
nuts with the insulation backing.
1" of isocyanurate is plenty.
NOW FOR THE DATA
.
JANUARY 21, AMBIENT TEMP AVERAGE 30 F, FLOW RATE 1 GAL/MIN
This data chart demonstrates the value of heat
stratification. Notice that the first tank to receive solar heated water is
always hotter than the last tank that returns water to the collector. No sense
in returning hot water to a hot collector. The difference in temp between the
first tank and the last tank varies between 10 and 20 degrees and the flow rate
for the entire system is about 1 gallon per minute. You might expect a higher
flow rate to extract more heat and it does BUT not as much as you might expect.
The turbulence created with high flow rate. diminishes heat stratification by
mixing hot and cold water.
Unfortunately I only have 3.5 hrs of sunlight to work with on a good day.
Shadows start creeping in after 12 noon so I'm only getting half the sunlight
possible without all these shadows.
FEBRUARY 3, AMBIENT TEMP AVERAGE 30 F, FLOW RATE 3 GAL/MIN
Sorry my probe slipped off the last tank for this test so we won't be able to observe the temp differential between the first and last tank, however since the flow rate of the pump has been increased I would imagine that the differential has dropped.
At 7 AM the temp inside the collector was 20 F by 9 AM it reached 100 F and 130 F by 1 PM. Without water flowing through the collectors I'm sure the collector temp would exceed 200 F BUT we're not interested in stagnation temp right? Without the shadow problem I believe the tank temp would have reached 130 F. Anyhow we cant measure what we don't have so let's measure what we do have.
For this calculation I will assume that the
average heat energy from the sun on 1 square meter at this orientation on Long
Island at this 3.5 hr time interval was 2,000 BTU's/ m2/BTU.
Without a pyranometer it's impossible to be sure. BUT from the information
shared with Gary and Richard I believe this estimate to be close.
OK
ENERGY AVAILABLE = 7 m2 x 2,000 BTUs x 3.5hr. = 49,000 BTUs
ENERGY COLLECTED = 120 X 8lb. X 32 F = 30,420 BTUs
EFFICIENCY = 30,420/49,000 = 62%