Can a chainsaw be ever powered by a builtin solarpanel?
Miroslav Kolář, February 7, 2016
In the Jan. 13, 2016 issue of The Bird's Eye (www.thebirdseye.ca) there was an interesting article "The coming of the Solar Age" by Greg Ross. He expects that photovoltaic (PV) solar panels will in the future become not only much cheaper and ubiquitous, but also so efficient that "we each will have just one solar panel the size of a tablet to power our home, car, and all electrical devices we own," and that "a small solar panel built into your chainsaw and other tools will allow you to use the tool nonstop all day, every day, just like today's solar calculators."
PV panels will no doubt continue to become more efficient and affordable, and I can't wait till the solar age comes in earnest. However, I was sceptical about the expectation that a small solar panel will sometime be able to power a chainsaw. The limitation is not so much the efficiency of the PV panels as the fact that the solar radiation is not that much concentrated. One has to harvest solar radiation from an area proportional to the power requirements of whatever is to be powered. The power requirements of a simple calculator are almost zero. I have not found specifications for any more recent calculator, but already in 1979 there was a Sharp calculator powered by only 0.0002 W (Watts). Today's calculators most probably require even less than that. But let us stay with 0.0002 W. An electric chainsaw needs at least 1,000 W to do any useful work. That is 5 million times more than a simple calculator. Therefore, to power a chainsaw directly by sun, one has to collect solar radiation from an area 5 million times larger than the one needed for a solar calculator. I have a calculator with a solar panel with an area of about 3 cm². Multiplying this by 5 millions gives 1,500 m². My calculator's little solar panel may still have the efficiency of only 5%. The maximum possible theoretical efficiency of a solar panel is 95%. For such a panel 1,500 m² would decrease to 79 m². This rough estimate of the solar panel area needed to produce a power of 1kW cannot be considered an authoritative figure because my solar calculator's panel may be overdimensioned.
It is not difficult to calculate a much more exact size of a solar panel needed for any power requirement from the data on the amount of sun radiation reaching the surface of the Earth that has been measured all over the Globe for decades. I have found, and will demonstrate in some detail below, that there will never be a chainsaw powered fully by a small builtin solar panel. On the other hand, slower lightweight vehicles powered directly (and only) by solar panels are possible, although with a surface area of quite a few square meters.
First a brief physics refresher: Watt is a unit of power, or rate at which energy is delivered (or work done). Thus Power = Energy/time, and one of the units of energy is Ws (wattsecond) or Wh (watthour; 1 Wh = 3600 Ws; 1 kWh = 1,000 Wh). Another name of Ws is Joule (J). And 1 Ws = 1 J = 0.239005736 Cal (Calories, familiar from the packaged food nutrition facts tables). And for the units of power, 1 kW = 1.35963 hp (metric or German horsepower, or PS DIN), and 1 kW = 1.34102 bhp (British/American or break horsepower).
The solar energy influx at the top of the Earth atmosphere is 1,370 W/m² (Watt per square meter – measured on a surface perpendicular to the Sun rays). Some of it is absorbed in the atmosphere, and so at the sea level one only gets about 1,000 W/m² (or 1kW/m²), and that is the maximum amount when the Sun is overhead at noon and there are no clouds. (This maximum solar input is assumed in the rating of PV panels. Thus if you buy a 100 W panel, you will get a 100 W output only when the panel is irradiated with1kW/m². From the panel rating and its surface area, one can thus calculate its efficiency.) The actual amount of available sun energy varies with time of the day, panel orientation, weather, and the season of the year. Because of this variability, it only makes sense to measure for a given place on the Globe, the total energy absorbed by a unit area over a longer period of time, given in units of kWh/m². Daily doses are usually averaged over each month or over the whole year from data collected over many decades.
Natural Resources Canada (http://pv.nrcan.gc.ca) has a map of this "mean daily global insolation" for different orientation of the absorbing surface, averaged annually or monthly. The largest insolation is for a panel that tracks the sun all day (2 axis suntracking), the smallest for a horizontally positioned panel. It is a colour coded map, where each colour corresponds to a range of values. NASA published exact values for various cities around the globe, but the ones I found are only for the horizontal position. NASA's values for Nanaimo and Vancouver (http://goo.gl/zn3UQV) are identical, and almost the same as those for Victoria. And they very well correspond to the upper bound of the NRCAN ranges for the horizontal panel position for the southern Vancouver Island. Thus we can also use the corresponding NRCAN upper bound values for the 2axis tracking. The largest daily insolation occurs in July, the smallest in December. In Table 1, the average daily insolation values (and the sunrise to sunset period) from the above sources are copied for these two months and for the whole year:
Table 1. Daily insolation for southern Vancouver Island (kWh/m²), and sunlight duration (hours)
Period:  All year average  July average  December average 
Horizontal panel  3.31 *  5.75  0.86 
2axis suntracking panel  5.8  9.0  2.5 




Sunrise to sunset (hours)  12  16  8 
* The 'All year average' for the horizontal panel was obtained by the averaging the NASA values over all 12 months, assuming the average number of days in February to be 28.25.
In Canada, only the Prairies and Southern Ontario have higher annual insolation than V.I. The closer one gets to the equator, the larger the insolation. Deserts and oceans in the tropical region can get on average daily insolation of more than 7.5 kWh/m² on the horizontal surface.
Table 2. Maximum theoretical output of a solar panel on the southern Vancouver Island (W/m²)
Period:  Annual average  July average  December average 
Horizontal panel  262  341  102 
2axis suntracking panel  458  534  297 
Period:  All year  July  December 
Horizontal panel  3.82  2.93  9.80 
Suntracking panel  2.18  1.87  3.37 
In the end, lets turn our attention to cars and trains:
There already exist solar “cars” without batteries powered by currently available panels. But they look more like fourwheel bicycles with a canopy, like this one in the sunny southern California, https://goo.gl/YFfrnn, running in full sun at16 km/h on a level road on a power of 720 W, produced by 920 W rated panels (thus they are getting there close to the maximally available insolation of 1 kW/m² at sea level).
Higher speeds require more powerful motors, and normal size car roofs are not large enough to accommodate panels to fully power for example a 10 kW motor even in the summer at this latitude (29.3 m² is the panel area needed for 10kW with the ideal 95% efficiency panel). One would definitely never be able to power directly by solar panels a Tesla S car that has a 290 kW motor (overdimensioned mainly to achieve an extra high acceleration).
More practical solution is to use rooftop panels as an auxiliary energy source together with batteries that are being charged between trips from other sources. Many groups have been working on this concept for some time. Since 1987 there has been an annual 3000 km Darwin to Adelaide (route going through another sunny desert) solar car race “World Solar Challenge”, http://goo.gl/KOXgJi. Competitors can still fully charge their vehicles from the grid before the start, and in some categories also in the middle of the race.
Ford CMAX Solar car has a 1.5 m² 300 W panel, and Ford is working on an interesting solar light concentrator to charge this car through its roof panel between the trips: https://goo.gl/zn27ek.
Honda has been testing a solar charging station for its electric vehicles since 2010 (a six hour solar charge is needed for a160 km drive).
One of the first demonstrations of a solarpowered car was in 1960 when an 1912 Baker electric car was fitted with then available solar panels: https://goo.gl/BZadJT. In this experiment 10 hours of solar charging enabled one hour of driving at 32 km/h.
SOLAR TRAINS?: Train is my favourite means of transportation that I miss in Western Canada, and so I was also interested to find how much solar power can be harvested from the roof of passenger trains and how it compares with their power needs. I did it for three cases for which I could quickly find all relevant specifications. All panel areas below are again based on the ideal 95% panel efficiency.
Amtrak18 coach electrified train: The coaches are 85 feet long, roof area of one coach is about 78 m². This area would collect about 20 kW of solar energy on average throughout the year (78 divided by 3.82 of the last table; maybe a bit more than 20k as it operates south of us). The electric locomotive is somewhat shorter, and so its roof could harvest another about 15 kW. That would give a total of 375 kW for the whole train, which is just a small fraction of its present locomotive's power input of 6,400 kWh. Such high power is needed to accelerate the train to 200 km/h in less than 8 minutes.
British Rail Class 466 Networker: Two coach electric multiple unit (EMU) with top speed of (only) 120 km/h. The total roof area of both coaches is 117 m², collecting thus on average about 31 kW (assuming similar insolation in Britain as here). There is just one motor in the whole unit, rated 600 kW, which is again much bigger than what the panels could ever in the future provide.
Liverpool Overhead Railway (historic). Built in 1893, it was using the first ever built EMUs. Each lightweight coach had one motor with power input of 45 kW (initially; over the about 60 years of operation of this railways, the motors were replaced by more powerful ones, first a 52 kW, then a 75 kW). Roof area of each coach was 42 m² and could thus collect on average about 11 kW, still only a 1/4 of the motor power. Even for such a simple, lightweight, slower train, solar panels would never be sufficient.
The conclusion: solar panels on train rooftops could only be an auxiliary power source for trains. The main solar generating stations (best in combination with wind turbines for night travel, unless cheap storage of solar electricity is available) would have to be put on the ground along the tracks. For this, all the tracks need to be electrified, as already done in Europe, except for some local routes.