by James A. Bowery
July 8, 2006
There is a great need to radically decrease the per person ecological foot-print of developed economies, world-wide -- a need to radically increase carrying capacity while reducing impact on high biodiversity ecosystems such as the Brazilian rainforests or continental shelf fisheries, and reduce concerns about global warming. There may be an economic option that uses sea water pumped to desert areas powered by the fact that ground level temperatures are much higher than temperatures at high altitudes. Indeed, it would dump greenhouse heat to space for its power while producing biodiesel, electricity, fish, fresh water, salt and real estate -- all in quantities demanded by developed-world populations -- without adding to, and possibly even sequestering, greenhouse gases.
Proposals for solar updraft towers have typically assumed that they would be single use structures: solar to electricity via heat differentials between high altitude air and ground level greenhouse-enclosed air. The resulting system has marginal economic value.
Something which would radically enhance the value of the solar updraft tower power structure is to use the greenhouse area for algae ponds to add biodiesel, water, fish and salt production to the production of electricity normally envisioned.
An objection to this combined use of the solar updraft tower is that the heat of vaporization lost during evaporation will translate into a lower temperature differential between ground and exhaust at the tower head. However, this ignores the recapture of that heat upon condensation -- a phenomenon that drives powerful natural phenomena such as thunderheads. The main problem, and it is a difficult engineering problem, is constructing an appropriate condenser at the top of the updraft tower. (One approach to the condenser problem may be to use Floating Solar Chimney technology as an alternative or in addition to the fixed tower).
Doing so brings the proposal from marginally viable to viable, with a net present value, primarily from live fish production, of $3.5 billion per system, thereby allowing for far higher capitalization and/or return on investment.
Let's start with just the value of algae biodiesel:
The greenhouse area required per solar updraft tower of the reference design is:
(pi * (5km/2)^2) ? hectares
= 1963.49 hectares
producing peak power of 200MW via a 1km tall tower.
We now add to this the production of algae biodiesel:
The UNH estimate for algae biodiesel production is 1 quad per 200,000 hectares. Let's assume only half of the area of the solar updraft tower greenhouse would be available for production at any time (the other half would be used for ponds that buffered heat for the inner ponds, produce fish, provide additional evaporative surface for desalination and provide recreation for residential areas at the outer rim).
That gives us:
(1963.49/2)hectares/tower;200000hectares/quad ? towers/quad
= 203.719 towers/quad
Or about 200 towers per quad of biodiesel.
We can now calculate the biodiesel per tower:
7.2gallon/1e6btu;200tower/quad ? gallon/tower
= 3.5998E+07 gallon/tower
or about 35M gallons of biodiesel per year per tower.
At $2/gallon for wholesale diesel, this yields $70M biodiesel revenue per year.
Now for electrical revenue:
At an average rate of sold production only 1/2 (100MW) of peak capacity (200MW), electrical production per tower per year, is:
100MW;year ? GWh
= 876 GWh
100MW;year;30$/MWh ? $
= 2.628E+07 $
or about $25M electrical revenue per year.
Interestingly, the biodiesel revenue is nearly 3 times the electrical revenue of a solar updraft tower!
200*200MW or 40GW electrical peak capacity is produced per quad of biodiesel.
Further that same UNH document estimates 19 quads to replace all transportation fuel in the US or 3800 towers, which would also produce 3800*200MW or 760GW or .76TW of electricity.
Current winter capacity in the US is about 1TW. So this cannot replace the entire US peak capacity but peak loads would probably be reduced under this system. Moreover, 3/4 is very close to 4/4 by engineering standards, so we can afford the luxury, in this instance, of assuming some innovation in electrical conservation technologies targeting reduction of peak load.
For reference, 3800towers at 1963.49hectares/tower would require:
3800towers;1963.49hectares/tower ? hectares
= 7.46126E+06 hectares
or about 8 million hectares or close to 30,000 square miles -- a figure that cross checks with the UNH figure of 15,000 square miles of optimally productive algae ponds which we are assuming are only half of our land area due to needed additional greenhouse warming area.
An additional advantage of this approach is that the relatively constant wind velocity and direction through the greenhouse disk would allow for the efficient use of wind for driving the algae raceways.
Now for desalination:
We're going to assume the algae ponds are saline, growing a marine species like CCMP647, and that about half of them are not producing biodiesel. These ponds would be out of algae production but would still be providing water for desalination, a market for the residual salts, live fish, climate control and residential real estate value.
Let's assume that out of the 8 million hectares, half of which is growing algae at reasonable efficiency and therefore providing 4 million hectares of evaporative surface, an additional 2 million hectares are in reasonably efficient production as evaporative ponds, some of which is salt production and some of which is fish production, for a total of 6 million hectares of evaporative surface. Then let's assume the additional difficulty of evaporating from saline cancels that gain out leaving us back at 4 million hectares equivalent fresh water evaporative surface. Using "Open water bodies in the Phoenix area evaporate at about 6.2 acre-feet per year (about two million gallons) per year for each acre of surface area."
We get:
2*10^6gal/acre;4000000hectares ? gallons
= 1.97684E+13 gallons
or about 20Tgal per year.
Estimating total US demand:
132gal/person/day;300Mperson ? gallon/year
= 1.4454E+13 gallon/year
or about 14Tgal per year.
The entire US requirement for fresh water can be approximately replaced with the desalinated water from the solar updraft towers.
Each tower's water output:
1.97684E+13 gallons/3800 ? gallons
= 5.20221E+09 gallons
and at a penny a gallon (remember this is high quality, nearly distilled, water):
1.97684E+13 gallons/3800;.01$/gallon ? $
= 5.20221E+07$
or about $50 million/year in water revenue. In all likelihood this would be much higher given markets for the distilled water could be found.
Salt is about $25/ton at the mine mouth.
And the ratio of sea water to salt mass is about 65:1 so the revenue from sea salt is:
25$/ton_salt;65ton/ton_salt;tonm/m^3;5.20221E+09 gallons ?
= 7.57404E+06 $
Or about $8 million/year in salt revenue. Perhaps this can be brought up by arranging radial evaporative ponds to fractionally crystallize higher value salts and accounting for the elimination of a return pipe for waste brine, but from salt value alone it seems barely worth the investment.
Now to live fish:
If the algae is 50% oil, and extraction of the oil isn't total, we can conservatively assume the mass of oil-depleted algae will approximate the mass of biodiesel:
0.827 g/ml;3.5998E+07 gallon ? tonm
= 124223 tonm
Trophic losses in aquaculture algae grazers are about 1/3 and the price per kg of live fish at the producer is conservatively $2/kg:
124223 tonm*.67;2$/kg ?
= 1.51009E+08 $
Or about $150 million/year in wholesale live fish revenue. This is a really big deal! Its as much as the electricity, biodiesel, water and salt production combined!
Totaling up yearly revenues:
$150M for live fish
$ 70M for biodiesel
$ 50M for fresh water
$ 25M for electricity
$ 8M for salt
$303M TOTAL REVENUE
Discount 20% for operation costs ($60M) and the yearly profit available is $240M/year or $20M/month.
A profit stream of $20M/month at 6% interest over 30 years has a net present value of $3.5 billion.
This compares very favorably with the estimated construction cost of the reference tower of $500M to $700M, which, of course, will have to be increased to account for the addition of a condenser to the tower, pond construction, centrifugal algae harvesters, boidiesel equipment, aquaculture equipment and brine transport systems. Indeed, the construction estimate for the reference system can be quintupled and still be financially sound if the already moderate technical risks are reduced.
Thus we have a system that is potentially profitable in the early stages of deployment, and that provides self-sufficiency in terms of high quality protein and water, with industrialized-nation levels of energy in the form of electricity and biodiesel -- all in an ecological footprint of 1/100 gha percapita. The primary remaining barriers to providing a standard of living comparable to the current US level are in secondary nutrition sources such as produce (fruits, vegetables, etc.). As it turns out, the waste product from fish processing is a high grade fish emulsion fertilizer for such produce, and can support high density hydroponic gardens grown in the aquatic recreation areas and even residential areas at the outer rim. If this proves insufficient, the expanded ecological footprint to support a wider variety of produce is unlikely to more than double the total footprint, and we are still looking at a 1/50 gha percapita -- resulting in the entire ecological footprint of the US population being something like the size of Florida (rather than half its size as is South Carolina). Of course, this ignores the potential to let people harvest naturally occurring produce from the greatly expanded wilderness areas resulting from the contracting ecological footprint. From a tourism and recreation standpoint it is precisely such wilderness areas that provide expanded opportunities for people to live-out hunter-gatherer behaviors that seems to drive so much of the tourism and recreation industry.