Many farmers have substantial heating costs for their greenhouses during the winter. In some areas farmers are resorting to burning coal to reduce heating costs due to the rising cost of fuels such as heating oil and natural gas. Fortunately, there is a way to substantially reduce heating costs using cogeneration technology. Natural gas is among the more common fuels for cogeneration since it is both a source of fuel for power producing engines and can provide usable heat captured from the exhaust stream. Furthermore, out of all the fossil fuels it is the least polluting, which makes it a better choice than, say, coal.
Natural Gas Cogeneration Cost Analysis
There was an article in my local newspaper that mentioned the possibility of using waste heat from a natural gas generator to heat greenhouses, and then selling the surplus power back to the electrical grid (cogeneration). I support this as a short-term solution, but I feel it’s preferable to sell power back to the grid only when it comes from a renewable source. But nevertheless, if it helps offset some of the power generated from coal power plants then it is (temporarily) a good thing.
So here are some numbers to consider if you want to purchase a generator powered by natural gas, sell electricity back to the grid, and use the waste heat to warm your greenhouse.
According to the article, the average Ontario greenhouse requires 9,500 gigajoules (9,500 x 109 J) of energy per acre every year, for heating. We want to size the generator so that the waste heat is sufficient for the average greenhouse.
Some of the larger commercial scale natural gas generators I could find were 150 kW models, produced by GUARDIAN Elite (see link at the bottom). The figure below shows a picture of one of their 150 kW natural gas generators.
Looking at their product spec sheet I see that for maximum rated power output (150 kW) the natural gas fuel consumption is 58.4 m3/hr.
Now, the energy density of natural gas is about 37,000,000 J/m3. A reasonable approximation is that half of this energy ends up as usable waste heat.
So, usable waste heat = 18,500,000 J/m3
The total number of cubic meters of natural gas needed is 9,500 x 109/18,500,000 = 513,500 m3
To consume 513,500 m3 of natural gas, the number of hours that the generator needs to run is 513,500/58.4 = 8,800 hours
Now, 8,800 hours is one year of operation. However, let’s say that no heating is necessary in the spring and summer months. So let’s assume that heating is only necessary during the fall and winter, which is about 6 months of the year. So we would need two generators instead of one, both running a total of 8,800/2 = 4,400 hours every year.
For each acre of greenhouse we would need two 150 kW natural gas generators, running non-stop for 6 months of the year. So for 7 acres of greenhouse we would need 14 generators (as an example).
The cost of each generator is about $30,000 (ref: http://www.aapower.com/naturalgas_gen.php). For each acre, that’s equal to $60,000 ($30,000×2).
Now let’s calculate the heating cost per acre.
In my area, the cost of natural gas is about 42 cents/m3. Therefore, the annual heating cost per acre is 513,500×0.42 = $215,700.
But since we are also using these generators to supply electricity back into the grid we can offset this cost substantially.
In my area the cost of electricity use is 15 cents/kWh. Let’s assume this is the amount the utility company would pay us to supply power to the grid.
Therefore, using the rated power of the generator, the total money back is 150×0.15x4400x2 = $198,000, per acre annually.
This means that we would only have to pay $215,700 – $198,000 = $17,700, per acre of greenhouse, for heating, every year. Clearly, this is a huge reduction. In fact, you can even end up not paying anything (or making a profit) if you have a lower cost of gas or a more efficient generator. That said it’s good to use as large a generator as possible, because it tends to increase the efficiency, and the economics of scale work in your favor.
Lastly, the cost per acre of heating the greenhouse without the use of cogeneration is simply $215,700/2 = $107,850, because we only need half as much natural gas if we are using it solely for heating.
To summarize, for each acre:
In your first year of use you would pay a total of $60,000 (generator cost) + $17,700 = $77,700. That’s a savings of $107,850 – $77,700 = $30,150 in your first year (using two 150 kW natural gas generators). And for each year thereafter you would save $90,150 ($107,850 – $17,700).
The upfront generator cost would pay for itself in two years.
Now there are issues like maintenance which add to the cost of course, but you’re still well ahead.
An important first step is to find out about grid tie-ins from your local municipal office. A grid tie-in is an installation that enables you to supply power to the grid.
A company in Kingsville, Ontario, Great Northern Hydroponics is doing what I just described, but on a much larger scale. They are using four high-efficiency GE Jenbacher engines (http://www.gepower.com/prod_serv/products/recip_engines/en/index.htm), which produce 12 megawatts of electricity. The exhaust heat is captured in water and transferred to the greenhouse, and the surplus electricity sold to the grid. In addition, the generator exhaust is routed via tubes that circulate near the stems of the tomato plants, exposing them to carbon dioxide. The tomatoes utilize the carbon dioxide to grow faster. Natural gas emits mainly water and carbon dioxide when burned, making its emissions pure enough to deliver directly to the plants.
Biomass Cogeneration Cost Analysis
There are alternatives to fossil fuels, such as burning biomass. For example, switchgrass is an ideal plant for its biomass potential due to its high energy yield and fast growth rate. It can grow in poor soil and climate conditions and requires little fertilization. It can grow up to 10 feet tall. Upon harvesting it can be converted to pellets, and then used as a fuel for heating in specialized gasifier stoves, or as a heat source for electricity generation in thermal power plants. The figure below shows a picture of switchgrass.
It’s much more efficient to burn a biomass and use the heat directly, then convert it into a liquid biofuel such as ethanol, to run vehicles. Consider that when used as a source of heat directly you are able to use all the energy it releases as it burns. But when converted into a biofuel for your vehicle, at most 20% will be converted into the useful mechanical energy that will power the vehicle. In addition, the conversion of biomass to biofuel can be energy intensive in terms of the resource and power consumption of the facility which produces the liquid biofuel. This can further reduce the all important ratio: (Useful Energy Output)/(Energy Input) – ideally this ratio is much greater than 1. For switchgrass pellets used as a heat source, this ratio is approximately 20:1 (ref: http://www.nrbp.org/papers/034.pdf)
The main advantage of biomass is that it’s carbon neutral, meaning it absorbs and releases the same amount of carbon dioxide during its growth and when it’s burned.
Performing a similar calculation as before, let’s consider biomass, in particular switchgrass as a source of energy. It costs about $7 to $8 per gigajoule (ref: http://www.gov.mb.ca/agriculture/crops/forages/bje01s01.html).
Using the 9,500 gigajoules from before, the annual heating cost per acre of greenhouse is therefore $66,500 – $76,000. Compare this to the $17,700 you would be paying annually using the natural gas cogeneration. It’s quite a difference in cost. But we can take this one step further.
Biomass can additionally be used for electricity generation, just like natural gas.
A Stirling engine (http://en.wikipedia.org/wiki/Stirling_engine) can use any source of heat to run, which makes it suitable for biomass. Using a similar set up, it can power a generator and feed power into the grid, with the waste heat used to warm the greenhouse.
Like before, let’s assume that half the energy input into the Stirling engine generator goes into usable heat that can be used to warm the greenhouse. This means that we need 2×9,500 = 19,000 gigajoules of biomass. Using switchgrass, this amounts to a cost of $133,000 – $152,000, per acre every year.
Let’s further assume that the Stirling generator has an efficiency of 25%, meaning that 25% of the energy input is converted into electrical power.
Therefore we have 19,000×0.25 = 4,750 gigajoules of electrical energy that feeds into the grid. This is equal to 1,320,000 kWh.
With a 15 cent/kWh return, the payback is 1,320,000×0.15 = $198,000, per acre every year.
So you would actually be making a profit of $46,000 – $65,000, per acre of greenhouse every year, using Stirling generators running at 25% efficiency, with switchgrass as a source of energy (at a cost of $7 to $8 per gigajoule). And the best part is you aren’t adding any carbon dioxide to the atmosphere. It’s win-win.
The figure below shows a picture of a Stirling engine generator.
It remains to calculate how many 55 kW Stirling generators are required per acre of greenhouse to satisfy the heating requirement.
As stated previously, the engines would operate about 6 months out of the year. This equals 4,400 hours of non-stop operation. This amounts to a steady power output of 4,750×109/(4,400×3,600) = 300 kW. So you would need more than one engine. In fact, you would need six 55 kW Stirling generators per acre. That’s a lot. However, Stirling Biopower is currently working on producing more powerful units.
Now, the cost of a Stirling generator can be high. At a cost of about $70,000 per 55 kW unit, this equates to $420,000 (per acre) for 6 units (ref: http://www.dleg.state.mi.us/mpsc/electric/capacity/energyplan/alttech/Stirling%20engine%20characterizations.pdf). But given the profit of $46,000 – $65,000 (per acre), it would pay for itself within 10 years.
An advantage of Stirling engines is that they are very robust, and can operate for a very long time with little maintenance. And costs are bound to come down as Stirling technology evolves and demand for alternative energy increases. Government incentives (rebates) should be explored as a way to offset some of the upfront cost.
Lastly, let’s calculate how many acres of switchgrass would be needed.
From before, 19,000 gigajoules is roughly equal to 1000 metric tons of switchgrass (19 gigajoule/ton). This amounts to 100-200 acres of growth needed per acre of greenhouse, for a years worth of heating and power generation (ref: http://bioenergy.ornl.gov/papers/misc/switchgrass-profile.html). That’s a lot of switchgrass! But there is plenty of lower grade farm land available which can provide the necessary yields. You can even use the same land that is occupied by wind turbines. Since wind turbines occupy little space at ground level, it allows additional renewable energy potential to be utilized without using additional land.
Farmers would stand to gain the most from biomass fuel, as they have the possibility of harvesting and processing the biomass from their land for their own use, such as in greenhouses. This can reduce costs and increase profits. And they would not be at the mercy of external market forces, as is the case with oil and natural gas.
Farmers can even opt to use the biomass from their land for pure electricity generation using Stirling generators to supply power to the grid. Of course a cost comparison would have to be done to see which use of the land would give the most economic return.
Referring to the above link, a reasonable yield of switchgrass is 6 to 8 tons per acre of land, every year. This is equivalent to 110-150 gigajoules. Assuming a Stirling efficiency of 25%, the annual power to the grid would be 25-40 gigajoules.
At 15 cents per kWh, the return is $1,000 – $1,700 per acre of land, every year. Clearly, only the larger tracts of land would give a good return. And if the land also happens to be located in a windy region, wind turbines can be installed to increase the return. Installing solar panels wouldn’t be a good idea because they tend to block out the sun, but wind turbines leave the terrain exposed to the sunlight needed for biomass growth.
Related Links:
http://www.guardiangenerators.com/Products/Commercial/Elite/Elite.aspx
All of this really makes geothermal sound like a more viable option.
I am looking 1 KW Stirling engine for solar concentrator.
IF any one know pls send me the Blue print or link,
thanks