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$2K Solar Space +
Water:
Thermal Performance
One Sunny Day in February
I plan to provide more performance
data over time, but here is at least one pretty sunny winter day to go by in the
mean time.
The plot shows the performance of the
system for Feb 21, 2011.
Black solid line -- Temperature of the
tank (closer to the top than the bottom) (F)
Dark blue solid line -- Collector inlet
temperature (F)
Red solid line -- Collector return
temperature (f)
Greenish line -- Solar intensity
measured in the plane of the collector (w/sm)
For this day, no hot water was taken from
the system, so all of the collector heat output went into heating the tank
water and raising the temperature of the tank plus some losses from the tank
during the day.
The tank started at 91F and was heated up
to 137F over the course of the day -- a tank temperature gain of about 46F.
The day was mostly sunny from late morning
until late afternoon. In the morning up to about 10:30 am there
were intermittent clouds.
The strange behavior of the sun line at
about 2:30 pm was caused by the pyranometer being borrowed for
another
test for a few minutes -- just ignore it.
The ambient temperature was 19 F at 10am
and got up to a high of 28 F in the afternoon.
There was some wind -- perhaps 8 mph on average.
You can tell when the differential
controller senses that the collector is cooling down just before 4pm and
shuts off the pump because the supply and return line temperatures take a
dive. This is because the sensors are mounted on the supply and return
pipes, which cool off quickly once the drain back empties them of water.
Heat Output for the Day
The net heat output for the day including
tank heat losses was about:
Heat Out = (170 gallons) (8.3 lb/gal)
(137F - 91F) (1 BTU/lb-F) = 65,000 BTU
This is the equivalent of 20 KWH
used in a 95% efficient appliance (like an electric water heater or
an electric furnace)
Or, about 1.1 gallon of propane
burned in a 65% efficient furnace or water heater.
The temperature rise for the water going
out to the collector is about 6F, which is good. A lot of
temperature rise makes for a higher average absorber temperature and more
losses out the glazing and lower efficiency. The 6F rise is
accomplished with the pump on the lowest of its 3 settings.
Pump power consumption for the day was (53
watts)(6 hours) = 318 watt-hours, or 0.3 KWH -- compare this to the 20 KWH
output -- the COP is very high.
Bear in mind that these numbers are for a
pretty sunny day, and not all days are sunny.
Rough Efficiency
Looking at the time period just after noon,
the temperature rise through the collector is 7F, and the sun is 1050
watt/sm.
The flow rate is 4.6 gpm.
The area of the collector is 93.2 sf.
Collector heat output = (4.6
gal/min)(60 min/hr)(8.3 lb/gal)(7 F) (1 BTU/lb-F) = 16,036 BTU/hr
Solar input to the collector = (1050 w/sm)(1sm/10.76sf)
(93.3 sf) ( 3.412 BTU/hr / 1 w) = 31,064 BTU/hr
Efficiency = (16,036 BTU/hr) / (31,064
BTU/hr) = 51.6%
This is a rough estimate.
It actually turns out to be very close to the efficiency of a Heliodyne
Gobi collector under the same conditions -- I would call this a ballpark
estimate, but it is a swell ballpark to be in :)
How
much of this goes for domestic water heating and how much for space?
Our domestic hot water use is pretty
low. I think less than 30 gallons a day for the two of us.
So, the energy to heat 30 gallons of water from 60F to 110F is
(30gal)(8.3 lb/gal)(110F - 60F) = 12,500 BTU.
This leaves 65,000 - 12,500 = 52,500
BTU for space heating.
In actual fact it may not work out so
that you supply all of the domestic hot water heat from solar and use
the rest of space heating -- you may choose to use more for space
heating and let your backup heater provide some of the domestic hot
water heating. A couple strategies for managing the
balance between space
heating and water heating are discussed here...
One thing to bear in mind when looking at
the performance of our system has to do with our collector orientation and
the mountains to the east. The mountains to the east block a bit of
the early morning sun this time of year -- not much we can do about this :)
Ideally we would clock the collector so that it pointed a bit west of true
south in order to make up for the bit of morning sun lost with more
afternoon sun. But, unfortunately, the south wall or our house
is actually pointed a little east of south. So, the combination of
these two things probably cost us an hour of sun a day this time of year.
Still, we on this day we got about 6 hours of sun, and that is fine, but if
your system is pointed due south and does not have these obstructions, it
should do a bit better.
Eight More
Days of Performance
Just below is a plot for eight days
from November 27, 2011 through December 3, 2011.
This was a period of mixed weather as
you can tell from the collector temperatures, which are a pretty good
indication of when the sun is shining.
One day of really good sun, several
days of part sun, and 2 days of almost no sun. Ambient temperatures
running mostly around 30F. We are less than a month from the winter
solstice, so 30F on a short winter day looks pretty good.
During this time I had the space
heating disconnected, so this shows the performance of the system in mixed
winter weather with domestic hot water demands only (no space heating).
The tank started the period down at
about 90F as a result of space heating taking it down to its lowest limit.
We are still working on what the best way for us to integrate the space and
domestic water heating.
Gary February 20, 2011