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solar vs. electric kiln drying schedules

5/12/14       
Heinrich Member

Website: http://www.heinrichspillmann.com

should I attempt to apply the same trying schedules on a solar kiln as for an electric kiln. How does the inconsistent heat source day/night affect the drying process. I finally have the possibility of installing a solar kiln but am uncertain about the heat and humidity controls.

5/13/14       #2: solar vs. electric kiln drying sche ...
Gene Wengert-WoodDoc

Solar kilns seldom have any controls. The fans and vents are operated manually--fans turned on in the morning after the kiln has heated up about 20F and off at night an hour or two after sunset. Vents are opened fully for fast drying species and kept closed most of the way for slower drying. When low MCs are need, then the vents are closed 99% to maximize heating. You should read the instructions that come with the VA Tech kiln design or the Wisconsin design. All are the same and based on what I first published in 1978.

The cooling at night is very beneficial...it provides for 100% RH which then relieves the drying stresses (also called casehardening).

5/13/14       #3: solar vs. electric kiln drying sche ...
Heinrich Member

Thanks Gene, so how then do the drying times compare from a solar kiln to an electric kiln. Under ideal conditions along the east cost (upstate New York) how much longer on average would a solar kiln take then an electric kiln to reach the proper MC. Can you elaborate on this. This would be helpful to me, so as not to create unrealistic expectations.

5/13/14       #4: solar vs. electric kiln drying sche ...
Henco Viljoen  Member

Website: http://www.tskilns.co.za

Problem with pure solar kiln is that drying times is not predictable. I am busy building my second hot water hybrid kiln that utilizes a combination of solar water heating, heat pump water heating and de-humidification to
to run a controlled schedule for drying of Eucalyptus saligna at between 48-51 Deg C (118 F - 124F) and RH% starting at 60% going down to 40%. Estimated drying time will be less than 9 days from ± 50% to 12 % MC

5/13/14       #5: solar vs. electric kiln drying sche ...
Gene Wengert-WoodDoc

In the U.S., wood and natural gas energy is so inexpensive that it is hard to justify using solar or other forms, such as electricity, based on energy price considerations alone. In some cases, solar energy systems have a government subsidy, such as a tax credit or even an outright grant, which makes solar feasible.

Solar drying time depends on the amount of energy collected. During six months of the year, the roof collector at 45 depress will collect, on the average about 1000 BTUs per day per square foot using a cover that is 85% efficient (glass is better, but it is also breakable band heavy). That means that each square foot will evaporate 1 pound of water per day, on the average...at 100% efficiency (no building heat losses and no vent losses).

If we consider a 1500 BF solar kiln with 150 square feet of roof area, and we have oak lumber going into the kiln green and this means about 2200 pounds of water to be evaporated per MBF, and also assume that the kiln is overall about 60% efficient (probably a bit high), then the kiln has 3300 pounds of water to evaporate, which will require 3.3 million BTUs of energy. With the useful energy in the kiln at about 600 BTUs per square foot, that means will will have about 90,000 useful BTUs per day collected, on the average. So, we are looking at 37 days to dry this lumber. Now, as we move past Sepatember, the solar energy input drops (lower sun angle and fewer hours of daylight), so drying time is extended. By November, drying is really slow and stays slow until March, at which point is begins to increase again.

Can you run a business that only produces dry wood during four or five summer months? Oftentimes, the answer is NO, so then a DH unit looks very attractive. With DH, to save the expensive electrical energy, the kiln is usually totally enclosed and well insulated. The capital cost is a bit higher per year than a solar kiln, but the cost is spread out over many 1000s of BF, so the cost per BF is lower. Note that with this design, it becomes reasonable to consider a separate solar collector that would provide hot water heating. But, as mentioned, in the U.S., hot water or hot air heated with natural gas or wood is a lot cheaper than solar, electric and most other fuels.

Here is a comparison of providing 3.3 million BTUS (April 2014 prices)

Electric heat $145
Electric DH $72
Fuel oil $93
LP $76
Natural gas $28
Wood variable $39

We must also factor in the burner needed. With electric, none or small; DH uses a compressor; the others use a furnace of some sort, with natural gas being the cheapest. Wood looks cheap, but there can be fuel preparation and storage costs.

5/14/14       #6: solar vs. electric kiln drying sche ...
Dave Boyt

You might check out Gene's article on solar kilns in the current issue of Sawmill & Woodlot Management magazine. Thorough & well written.

Sawmill & Woodlot Management magazine

5/15/14       #7: solar vs. electric kiln drying sche ...
Heinrich Member

Website: http://www.heinrichspillmann.com

Thank for your help. I am planning to install this kiln off site, at a location with no electricity. Living in NYC real estate for solar kilns is hard to come by. I am looking into powering the fans with solar panels. Also because of the remote site I am hoping to be able to set it up such that I can leave it alone for up to five days without maintenance, since I might not be able to get to it during the week. This is a small kiln (4 x 10 feet or smaller) for my larger sculptural wood carvings. (see my website). It sound like that after some testing I should be able to find the proper setting between exposed glassing surface, the fan hours and vent openings so that I could leave it alone during the different stages. At least no risks of electrical fires... I have been pocking around in the Virginia Tech paper for Solar Kilns, but also of course the Oregon State University paper by Gene. Thank you Gene for all your freely accessible contributions and advice.


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