Goat Dairy Farm
Moderator:Bob
My family operates a goat dairy and I am trying to find a way to save some money on the heating costs of the milk rooms for the winter. We generate a lot of manure but because bedding material of straw and hay is included I wonder if C/N ratio would be ok. I have seen several designs for biogas generators on the site and I think I can fabricate one myself, but I cannot find an apropiate way burner to generate the heat required. Also becuse this would be mainly for night operation I am curious as to how to store the gas in sufficient quantities.
That's a good question. I don't have C/N ratio information on goats, but if it is similar to sheep, the info I have on sheep indicates a ratio of about 20:1. It also depends on what kind of bedding you use. The literature indicates that the C/N ratio of different straws and grasses can vary from about 20:1 to 80+:1. All in all, I'm guessing you won't have a problem, but you'll probably need to do some initial testing to know for sure.We generate a lot of manure but because bedding material of straw and hay is included I wonder if C/N ratio would be ok.
but I cannot find an apropiate way burner to generate the heat required.
I have no idea how big a system you might be talking about, but for a small scale system, probably the easiest might be to pipe it to a gas-fired hot water heater, then circulate the hot water to piping in a radiant floor system or hydronic unit heaters (and/or hot water storage tank -- see below).
As far as storage, again, it depends on what size of system we are talking about. One possibility might be to burn the gas as it is generated, and store the energy as heated water. This would require an appropriately sized insulated water tank, rather than gas storage (and compression) facilities.I am curious as to how to store the gas in sufficient quantities.
Bob, thank you for your reply
I did some calculations and I think I require about 40,000 btu/hr at peak load, so I was thinking along the lines of a thermostat controlled forced air system. Radiant water in the floor is not possible as the concrete slab has already been poured. A hydronic water system would be possible as there is already a hot water tank (electric though) in the room, however I think the cost of these might be a fair bit greater than a forced air system. Either way I would need gas storage capabilities as I would not think it possible to generate enough gas to run on demand from what is produced from the digester system
Thank you for your input
I did some calculations and I think I require about 40,000 btu/hr at peak load, so I was thinking along the lines of a thermostat controlled forced air system. Radiant water in the floor is not possible as the concrete slab has already been poured. A hydronic water system would be possible as there is already a hot water tank (electric though) in the room, however I think the cost of these might be a fair bit greater than a forced air system. Either way I would need gas storage capabilities as I would not think it possible to generate enough gas to run on demand from what is produced from the digester system
Thank you for your input
Have you calculated the amount of gas that you think you can generate from the waste material available? Is it enough to heat the facility at design load?
Assuming a peak load of 40,000 btu/hr, I might guess that, on the coldest day of the year, you might need, say,
40,000 btu/hr x 24 hrs x 75% = 720,000 btu/day
If your heater is 75% efficient, you would need:
720,000/0.75 = 960,000 btu/day input
At a heat content of about 950 btu/cu ft (of methane), you would need to store about 1,000+ cu ft of gas. (If you don't scrub it you would need about 1,600+ cu ft of biogas storage.) You could compress it, but unless you have cheap abundant (free?) electricity to spare, it probably wouldn't pay.
Now, here's an alternate scenario. Suppose you only have, say, 10 cu ft of gas storage capacity, and burn it in a water heater each time the storage bag is full, cycling the heater on and off. Let's say you can heat the water 30 degrees F, and circulate it through a fancoil heater. Water weighs about 8.33 lb/gallon. It stores 1 btu/lb/degreeF. The size of water storage tank you would need to store one full day's worth of heat at design conditions would be:
720,000 / (8.33 x 30 ) = 2,881 gallons.
At 7.48 gal/cu ft, this is about 385 cu ft of storage. The higher delta tee you can achieve, the smaller the storage requirement. If you could achieve 60 degrees F delta tee, for example, you could cut it in half.
Storing the energy as hot water rather than as a flammable gas is also a lot safer and easier to deal with.
Of course you wouldn't need to store a full day's worth of heat in either scenario, because you would be using some of it as it is generated. I'd guess that a 1,000 gallon tank (or 3-4 300 gal tanks?) would be a reasonable guess -- depending on a whole lot of stuff I don't know, about your local climate, building insulation, fuel costs, site conditions, etc...
Assuming a peak load of 40,000 btu/hr, I might guess that, on the coldest day of the year, you might need, say,
40,000 btu/hr x 24 hrs x 75% = 720,000 btu/day
If your heater is 75% efficient, you would need:
720,000/0.75 = 960,000 btu/day input
At a heat content of about 950 btu/cu ft (of methane), you would need to store about 1,000+ cu ft of gas. (If you don't scrub it you would need about 1,600+ cu ft of biogas storage.) You could compress it, but unless you have cheap abundant (free?) electricity to spare, it probably wouldn't pay.
Now, here's an alternate scenario. Suppose you only have, say, 10 cu ft of gas storage capacity, and burn it in a water heater each time the storage bag is full, cycling the heater on and off. Let's say you can heat the water 30 degrees F, and circulate it through a fancoil heater. Water weighs about 8.33 lb/gallon. It stores 1 btu/lb/degreeF. The size of water storage tank you would need to store one full day's worth of heat at design conditions would be:
720,000 / (8.33 x 30 ) = 2,881 gallons.
At 7.48 gal/cu ft, this is about 385 cu ft of storage. The higher delta tee you can achieve, the smaller the storage requirement. If you could achieve 60 degrees F delta tee, for example, you could cut it in half.
Storing the energy as hot water rather than as a flammable gas is also a lot safer and easier to deal with.
Of course you wouldn't need to store a full day's worth of heat in either scenario, because you would be using some of it as it is generated. I'd guess that a 1,000 gallon tank (or 3-4 300 gal tanks?) would be a reasonable guess -- depending on a whole lot of stuff I don't know, about your local climate, building insulation, fuel costs, site conditions, etc...