Solar Heating in January

Using the sun's energy to heat a house in January may sound crazy but as the oil reserves run low we may realize that burning oil to stay warm is not as crazy as burning non-renewable resources. A well planned, well insulated house with a south facing roof devoted to collecting heat with drums of water devoted to storing heat heat is a good start, but we'll still need to integrate the solar home heating system with a year round solar DHW system to be cost effective. Of course a back up heating/DHW system may be necessary, but oil burners are not. During those long cold intervals without sunlight a simple  wood burning stove can be used to bridge the gap between sunlit skies and long, dark, cold intervals of time.

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Modified Trickle Down Sun Shed on Long Island, NY

My little, experimental Modified Trickle Down storage shed has absolutely no back up heating system and minimal insulation so as you might expect I am not living in this shed.  Since the only source of heat comes from the sun the shed provides an ideal tool for solar thermal research. Storage drum temperature, collector temperature, ambient temperature, and sunlight intensity readings are sampled at 4 minute intervals throughout the month to track the rapid solar heat gain from the sun as well as the gradual heat loss to the environmental. 

 

Unfortunately almost half the possible sunlight  is blocked by a nearby oak tree, power cables and a neighbors maple tree, but despite these limiting conditions the data collected demonstrates the value of a Modified Trickle Down solar heating system. As a matter of fact unhampered direct sunlight is available between the hours of 9AM and 11AM so an objective analysis of the data may still be performed. Here is a bird's eye view of the actual sunlight striking the shed's glazing during the month of January, 2009.

As you can see from this graph not all days have an equal amount of sunlight available to them, but every day receives some sunlight. The question is how much sunlight is needed to turn the pump on and supply a net heat gain to the heat storage vault. According to the above plot the best days have a maximum sunlight intensity between 600 W/m2 and 700 W/m2. This translates to about 2000 BTU's/m2/hr. Examine the graph above and see if your analysis coincides with my finding of days that had little or no heat gain. 

Days without solar heating value: Jan 1,3,5,8,9,10,13,16,18,21,30,31.
The other days of the month do contribute to the net heat gain of the heat storage vault to variable degrees. Some days contribute more than others, but with a little luck sufficient heat may be collected to carry the system through difficult dark periods. OK now we are ready to examine the overall thermal data associated with ambient temperature, collector output temperature, collector input temperature and average storage vault temperature.

 

Pretty confusing, hey? I guess, but can you still see temperature patterns? First take a close look at the ambient temperature fluctuations during this month. Notice the ambient temperature reaches a high of 480 F on January 1 and a low of 50 F on January 17. The average ambient temperature for the month of January was about 290 F.

A  peak collector temperature of 1020 F was reached on Jan. 2 and a collector low of 20 F was reached on Jan.16 but collector temperature alone won't help us calculate heat gain because heat gain depends on flow rate, input and output temperature as well as sunlight intensity so how can we calculate the net heat gain of this MTD collector array with a 200 gallon multi tank storage system. For this we will need to take a close look at the average temperature fluctuations of the 200 gallons of water. To simplify the heat gain calculation let's focus only on the storage tank temperatures.

The top plot illustrates the rise and fall of temperature inside the storage vault.
The bottom plot illustrates ambient temperature readings from the north side of the shed.
    As you can see ambient temperature has little or no relation to the heat gain of the system, but you already know that, RIGHT? So let's focus on the top curve which is a plot of the average storage temperature fluctuations during the month of January. From this top plot observe a high of 950 F on Jan 2 and a low of 600 F on Jan 11. The sharp rise in temperatures take place when sunlight is available and the pump is on. Temperature drops occur when the pump is off and sunlight is not available.
    OK first let's first estimate the total solar heat gain for the month of January. Remember this shed is isolated from fossil fuel heating systems and the R factor of the storage vault is low so that heat is intentionally lost at a fairly uniform rate. The advantage of of this experimental energy independent system has to do with the predictable heat loss throughout the month. Let's take a close look at the 3 days of heat loss with no solar heat gain that occurred between January 5 and January 8 and you'll see what I mean.

 

 

First notice the consistent storage temperature drop of about 60 F per day with pump off. This heat loss will continue weather the pump is on or off. As a matter of fact heat loss will be even greater when the pump is on with hot water is flowing even though we observe a rapid temperature rise during these brief intervals of sunlight, but let's settle on a temperature drop of 60 F per day for now..

Second observe that the average storage temperature during January has been maintained at about 750 F
throughout that month. This means that the heat lost to the atmosphere is equal to the solar heat gained.
For the month of January 2009, the solar heat gain is:
60 F per day x 31 day x 200 gallons x 8 lbs/gal   =  297,600 BTUs

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COLLECTOR EFFICIENCY CALCULATION
USING STORAGE TEMP CHANGE

OK, now let's see if we can estimate collector efficiency. To do this we'll examine the available sunlight and hourly heat gain on January 16  between 10AM and 11AM.

Upon close examination of the above plot you will observe a flux intensity of 650 watts/m2 between 10AM and 11AM. This translates to about 2000 BTUs/m2/hr. Now let's take a peak at the temperature rise between these hours.

 

As you can se the outside temperature between 10 AM and 11 AM hovers around 170 F, but let's forget about the outside temperature and focus on the 50 F temperature rise of the 200 gallons of stored hot water.

The is a solar heat gain            50 F x   200 gal x 8 lb/gal        =   8,000 BTU/ hr 
The solar energy available        8m2 x  2000 BTU/m             =  16,000 BTU/hr

COLLECTOR EFFICIENCY        8,000/16,000                         =    50% 
There may be some discrepancies in this estimate but I do believe the margin of error is small. 

 

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As you can see from the above plots of flux density more than twice as much sunlight was available during the month of January as was available during the month of December and the heat gain in January was greater even though the outside temperature was lower.

The main point is that the amount of sunlight and the surface area of the collector array mainly determine solar heat gain. A 300,000 BTU heat gain to a partially shaded sunshed in the month of January may not seem like much but with an array four times larger ( which I recommend) the heat gain for the month of January on Long Island could easily be 2,000,000 BTU's. The results for February will be posted soon. Be sure pay visit again when the solar data from February is posted. I expect the energy harvest for this month to be greater.

Here is some thermal data from February 16, 2009. I'll let you do he calculations. Notice that more heat is collected on this February day than on a January 16. This is mainly because of less shadows on the collectors. ALSO notice the heat loss from the 200 gallons of stored hot water is more rapid. This is because the storage vault temperature is higher than it was in January and temperature differential is a major factor in the rate of heat loss. See what other information you can deduct from this data plot.

 

December Solar Heat Gain
MTD Data

MTD Solar Heating

MTD Solar Collector Kit