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Solar Power vs. A Diesel Generator
I love solar. I'm a second generation electrical engineer and Photo-Voltaic (PV) arrays (aka, solar cells) are solid state integrated circuits, so we're practically family. But one the required courses in school was engineering economics. Frightfully boring stuff really, but ultimately the basis for almost all real-world engineering decisions. As freshmen engineers we all think it'll be high-tech this and leading edge that, but come to find out on the job it's mostly about money. Ah, the dreams of youth. ;-) Solar power is (sadly) one such dream.
Today we'll design an off-grid power system. We'll leave out quite a bit of detail so the article will fit in one issue, but we'll include enough to make a valid economic selection between the two approaches. One system will be strictly active solar and the other entirely diesel generator based.
At the onset let me say that it would be very unusual for either system to be the best choice in practice. Generally a hybrid that includes both solar and a generator will be the most cost effective, but these two extremes are shown here to (1) make some key points, and (2) illustrate the analytical process.
Furthermore, the purchase of a new diesel generator is almost never warranted. Used diesel generators from surplus and industrial sources cost less than half as much as new. This would dramatically change the cost analysis shown below. For more information on purchasing used diesel generators see <http://www.cheap-diesel-generators.com>.
This article first appeared in an issue of Off-grid Online, once a publication of OffgridKnowHow.com. For more information on off-grid and homestead technology see <http://www.homesteadtechnology.com>.
Problem Statement
Provide full time power to a residence for a family of four without access to the utility grid. All modern appliances will be provided, but it can be assumed that the owners conserve power where possible. Most heating applications (air, water, clothes dryer, and cooking) are non-electric, although heating by forced air does utilize an electric blower. The one electric heater included in the design is a dishwasher, an appliance where no gas-fired equivalent has been found. A free-flowing water supply, needing no pumping, is assumed for simplicity.
Some Givens
- The cost of active solar panels is $ 5 per watt of capacity.
- The cost of diesel generation capacity is 65 cents per watt installed with tank.
- The cost of diesel fuel (off road, no federal tax) is $ 1.00 per gallon delivered.
- The fuel consumption for the diesel generator is 10KWH per gallon.
- A battery system is assumed so a charger and the electrical load of a charger is included in the generator/panel capacity.
System Capacity
The total of larger appliance loads could create a peak demand as high as 8 KW or more, but we will design for only 4 KW for affordability. The owners must use some care in scheduling their power use in return for a lower total system cost. Assuming conservative use of power, a 12 KWH per day total usage, 500 watt average continuous use, is achievable even with modern conveniences. Energy efficient appliances are assumed and those of us with three computers running 24/7 need not apply :-).
An inverter capable of 4 KW output will include a 4 KW battery charger as well. This charger will be driven by the generator or solar array so that power is available full time.
Generator Based Solution
The best use of a generator is to use it near capacity for as short a time as possible. Since we are designing for a 4 KW peak use, we will specify an 8 KW generator. Here's why.
Our usage of 4 KW is simultaneous with a a battery charger load of as much as 4 KW for a maximum generator demand of 8 KW. But the maximum charge rate and maximum usage will not likely occur together or for very long, so the more common operating load will be closer to 6 KW. Since the most efficient operating point for the generator is at about 80% of capacity, an 8 KW generator provides what we need.
Such a generator can be purchased new and installed with a fuel tank for less than $ 6000. At a usage rate of 12 KWH/day, the diesel fuel consumed on a daily basis will be 1.2 gallons for a cost of $ 1.20. The annual fuel charge is then $ 438. To this figure we will add $ 20 to account for periodic oil and filter changes (5,000 hours, or about every 4 years in this example).
Tabulating our total energy cost year-on-year we get the following end of the year energy costs:
yr tot cost 1 $ 6458 2 $ 6916 3 $ 7374 4 $ 7832 5 $ 8290 6 $ 8748 7 $ 9206Our ongoing per KWH electricity cost is a hair over 10 cents. When we factor in our $ 6000 initial investment and spread the cost over a ten year period (pretty arbitrary really, the system should last longer at this usage level), we get a fully burdened electricity cost of 23.7 cents per KWH.
Solar Based Solution
To size our solar system we must know the capacity factor (CF) for the location where our system will be sited. In North America values for CF vary from less than 15% in the north to about 25% in the south. The CF is the ratio of how much power is available compared to 'round the clock full solar load.
Here's how that works. If the sun was directly overhead for 24 hours straight, a 100 watt panel would produce a total of 2.4 KWH (24 x 100). Fortunately this doesn't happen (it would get a bit hot...). Instead, the sun is not up at all for part of the day (we call that night) and even when it is up, it isn't up all the way. The net result of that is the capacity factor which is usually given as an annual average. Obviously, summer is higher than winter.
Let's use a middle value of 20%, though my own location is closer to 14%. At a factor of 20% a 100 watt panel will produce .48 KWH each day. With our daily usage of 12 KWH, we will need a total of 25 panels rated at 100 watts each. At current prices this amounts to $ 12,500.
Since no one actually owns the sun, our "fuel" is free, and our initial investment is (theoretically) our entire life-cycle cost. As we did for the generator solution, let's spread the initial investment (plus the zero operating cost) over a 10 year period (yes, it will last longer, we'll get to that in a minute). Our energy cost, at 12 KWH per day for 10 years given an initial system cost of $ 12,500 is 28.5 cents per KWH.
Analysis
In raw dollar terms, it takes 14 years for the solar solution to pay for itself compared to the generator based approach. For every year after 14, the solar system accrues a savings of $ 458, or about 3.7% of system cost.
By the end of year 20, our 100% solar solution is winning hands-down compared to the 100% generator solution. Our aggregated cost of power is now 14.26 cents per KWH and continuing to fall.
All mechanical devices, generators included, will wear to the point of wearing out at which point they must be replaced. The choice of a diesel generator, a far more expensive alternative than a gasoline fueled unit, is made because of the far lower life cycle cost of diesel. A diesel generator that is well cared for should easily last 15 years with nothing more than cleaning, occasional adjustment, and periodic fluid changes.
The solar system can be incrementally expanded as usage requires and money allows. The generator based system must be sized for current and expected needs when installed. But then, at only 65 cents per watt, over-sizing a generator is an affordable option.
The further south we site our system, the better the solar solution performs. That's not a surprise. At far northern latitudes the solar system performs very poorly. Another no-brainer.
It is honestly incorrect to design the solar system for the annual average CF since the winter time CF is the controlling factor. All solar systems, active and passive, have this problem: just when you need it the most (winter) you have the least sunlight available. At a location with an annual CF of 20%, we can expect that the winter CF will be more like 15%, so our array will be undersized!
Special situations will control the choice of power system without an economic analysis. For example, if you plan on powering an arc welder, or running a machine shop, you need not consider solar for those loads. A generator would be the only option.
Conversely, there are locations that do not provide the initial and ongoing access required to place and supply a generator. These will be solar applications without question. Remote, unattended systems, even with good access, should also be PV.
Neither of the designs presented makes optimal use of technology. The best arrangement would use both solar and a generator. The generator is best suited to short duration bulk charging while the solar panels are best suited to long duration final charging. A system built with both generator and panels would have far fewer panels than a full solar system and would use far less fuel than a full generator system.
Where Goith Solar
In the early eighties, PV prices were predicted to be 50 cents per watt by the end of the decade. Right. Missed that one! The going rate at the time was $ 8 per watt. Today, twenty years later, it is $ 5. Improved, but not very promising. For perspective, we have seen a 100 fold increase in microprocessor speed in roughly the same period.
PV manufacturing is a mature industry. Prices can be expected to decline somewhat with volume, but what is really needed is a breakthrough in either device physics (leading to increased conversion efficiency) or manufacturing process (resulting in reduced cost at current efficiencies). I see neither of those on the near horizon.
Solar Does Not Equal PV
Not all solar is photo voltaic (PV). A number of folks have made good progress on solar driven engines. Some of these are already showing collection efficiencies that surpass PV. The conversion efficiency of PV cells has doubled since their commercial debut, but they still lag Rankine cycle heat engines (steam engines are Rankine cycle). There are commercial solar driven steam engines today that provide power for $ 2.5 per watt.
Solar driven heat engines lack the elegance of PV. They have near as many moving parts as generators and do make some noise (though not as much as a diesel!). They are by no means mature. Not only are these machines being improved, but different thermodynamic cycles are being tested that offer the potential to double the efficiency of the solar heat engine approach.
If we could get to $ 1 per watt from solar heat, the tradeoff between solar and petrochemical changes dramatically.
Hoorah for the Ugly Sister
Industrial diesel generators are plentiful items in the surplus market. Our design comparison was done using "new" costs to keep the fight clean, but I personally would not recommend the purchase of a new diesel unit where a good condition unit is available surplus. My own (10 KW) generator cost $ 750 and has been in use for nearly three years. I know others who have purchased similar units for $1000 to $1500. [To find out we did this see cheap-diesel-generators.com].
At surplus prices, the PV-based system would take 22 years to catch-up with diesel. But will a surplus generator last 20+ years? At just 4 hours a day, probably, but let's be a bit conservative and assume a life of only 10 years. If we assume replacement of the generator every ten years at a cost of $ 1500 and an initial cost of $ 500 for non-replaceable parts, we have the following year-on-year energy cost.
yr tot cost 1 $ 2458 5 $ 4290 10 $ 8080 15 $ 10370 20 $ 14160It is not until year 20 with the third surplus unit that we have as much invested in a generator as the cost of a new unit. It is also in this year that we surpass the cost of the PV based system. As can be seen, it takes a while to make up the difference between 15 cents per watt and 5 dollars per watt.
Renewable Diesel?
Diesel fuel is not the only fuel a diesel will run on. The addition of methane at the air intake allows the use of much less diesel, typically one third the normal fuel use, resulting in reduced cost and reduced pollution. The methane can be from the bio-gas output of a methane digester.
Or the diesel can be entirely replaced by so-called bio-diesel -- reconditioned friar oil from the local fast food outlet. Here too, cost can be reduced and it is claimed that vegetable oils burn far cleaner than petroleum products. Depending on which variation of several processes are used to process the used cooking oil, a gallon of bio-diesel can be produced for as little as 50 cents per gallon.
PV or Not PV?
I am not selling either one. Just do the math. Sometimes one will win hands down, sometimes the other. More often than not however, a combination is the best choice. But the ONLY way to know is to do the math.
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Want more? see http://www.homesteadtechnology.com