Steve Lapp's Solar-Assisted Prius
By Bill Moore
Conventional wisdom says that you can't practically run a real car on sunlight. Sure, college teams manage it for Solar Challenges, but how about something like a Toyota Prius? The prevalent viewpoint is that it just isn't worth the expense and the effort.
But Steve Lapp of Kingston, Ontario, isn't convinced, although neither is he under any illusion that he can run the car on solar energy alone. Instead, he wanted to know if adding a few square meters of solar panels to the top of his four year-old Toyota Prius would improve the car's fuel economy.
Lapp is no novice garage tinkered. He helped engineer the first vehicle -- a Dodge Caravan converted to electric-drive by UQM-- in Canada to run on an aluminum-air fuel cell. Operating as a range extender for the vehicle's Chloride lead-acid batteries, the vehicle drove 300 kilometers in 1989, though the cost of the fuel cell and problems with recycling it proved uneconomical.
What got him to thinking about solar assist were three relatively recent developments: high-efficiency solar panels that can be bought "off-the-shelf"; modern, high-power electronics; and very efficient hybrid vehicles that consume only 125-150 watts of electric power per kilometer.
The mono-crystalline photovoltaic panels Lapp is using are rated at an efficiency of 16.5 % and only became available commercially from SunPower in January of this year. His stainless steel, roof-mounted rack, which includes a streamlined leading edge and extends slightly over the rear window, can generate a total of 360 watt-hours of electric power from four 90 watt panels. It bolts into specially-designed brackets that fit into the existing roof channels in the first generation Prius.
"I chose the panels specifically because their dimensions match the width of the roof where the groove is very nicely and made for a very simple bolt-on structure", he explained.
But why not just put the panels on the garage or a parking structure? You'd certainly have more space for more panels. Lapp agrees, but notes that its only available to recharge the car's battery when the car is parked at that location. While there are weight penalties for carrying the panels and extra batteries with you, which impacts fuel economy, it offers the advantage of letting you opportunity charge regardless of where you are.
"The fact that you have PV on the car allows you to take the energy with you", he observed. That's energy that can be used to not only recharge the batteries but also run ventilation fans, various onboard electronics, even someday preheating the exhaust catalyst so the gasoline engine pollutes less during start up, he said.
He also made the case that you want to put a PV system only on vehicles that get used daily, in increase the capacity or usage factor of the investment. If a vehicle is only used occasionally, the "buffer" battery, as he calls it, gets charged and any surplus energy is essentially wasted.
He estimates a vehicle needs to be driven at least 10 kilometers (6 miles) daily to make the investment worth it.
Lapp's system, including panels, roof rack, 70 amp hours of lead acid batteries and associated power electronics, inverters and converter weighs 65 kilograms (143 lbs). For funding reasons, he opted to pretty much leave the Toyota NiMH battery system and its controls alone.
"The whole life of the hybrid battery is pretty much dependent on the charge management system that Toyota has built into it". Lapp has discovered that Toyota's charging system on the first generation Prius rarely charges above 65% SOC and never lets it drop below 45% SOC.
"So, I added extra batteries to take the PV energy and only when the car is running do I allow that extra energy to feed into 300 volt circuit on the car". Using specially-designed circuitry, Lapp can manually adjust the amount of watts that he can feed into the hybrid drive from his PV-charged battery pack.
"Typically, I am going to have that at 400 to 500 watts until the lead acid battery pack is depleted, and then I go down to whatever power is available from the PV system. So, this morning when I drove into work, the sun was shining, it was fairly early so I could only get a about hundred watts out of the PV system. So, once I'd depleted the stored energy in the lead acid batteries, then I was basically just adding what was available instantaneously from the sun".
What Lapp is discovering is that in city driving, the Prius runs further on its electric motor then it would normally, which means the gasoline engine comes on less and that translates into better fuel economy. Unlike electric, plug-in hybrids, which are designed to run on electric power only for 20-to-60 miles, Lapp‘s Prius functions exactly as Toyota designed it, only more efficiently in terms of the total gasoline burned.
Lapp estimated that he has about CAD$3,000 (US$2564.00) in the project, though he did have the batteries already. He said he paid retail prices for everything, including CAD$600 for a 600 watt inverter and around CAD$2000 for the PV panels.
He told me that his hardest technical problem was coming up with a way to generate 300 volts DC to match the Prius' voltage.
"There just isn't equipment out there that you can buy that produces 300 volts DC from the PV panels… It's really the voltage conversion from the PVs to the 300 volts DC that is the tricky part. I chose a very simplistic way of doing that because it's what's affordable, not because it's the best way technically. I chose to convert the PV energy initially to 120 volts AC then upped that to 345 DC, and then rectify that back down to 300 volts DC through a pulse width modulator".
Lapp concedes that power electronics design is the weak point in his repertoire of skills and he had to get help with the circuit design. While he now works as for St. Lawrence College in Kingston, Ontario, he had completed most of the design before starting his new job teaching a new course renewable energy systems. Prior to taking the job with St. Lawrence College, he ran his own company, Lapp Renewables, engineering and installing renewable energy systems.
Lapp talked briefly about his involvement in the aluminum-air fuel cell project in 1989, explaining that while the experiment proved it is technical feasible to run a vehicle using this type of fuel cell, it's not very practical or economical, in large because of the need for very pure aluminum and the fairly rapid degradation of the stack membrane after just a few weeks of operation.
I asked Lapp if he had any data on how much the solar panels contributed to the performance of the car. He explained that he's not had enough time to collect any firm data, but it's his impression that it is making an important contribution.
"I have hardly been able to do any comparison runs like with the system on and with the system off. I do know that on the few runs I've made, I've been able to meet or exceed the very best mileage that I've ever gotten in the car under ideal driving conditions in the past. Just on my drive into work, I‘ve been able to exceed those very best conditions, which was down to about 3.9 liters per 100 kilometers". He quickly calculated that to be equivalent to 58 US miles per gallon. This compares to his normal commuting fuel efficiency which averaged about 50 mpg.
While these estimates are admittedly "loose", he does have a better feel for what the system should do theoretically. He calculates the Prius uses 125 watt hours per kilometer in an urban driving environment. Given the amount of solar panels on the car and the current sun angle in eastern Ontario, he estimates the batteries will receive about 800 watt hours of energy over a four hour period. Dividing this by 125 watts hours translates into just over 6 kilometers of solar electric-assisted, gasoline-free travel. If your total commute is 20-30 kilometers, he explained, this can represent an additional 25 percent reduction in fuel consumption.
Lapp doesn't see the need for any big technological breakthroughs in order for carmakers to someday offer this as an option. He figures in volume numbers, it would cost maybe $1000, in part because much of the electronics are already onboard the hybrid and just need adaptation.
"I think what's needed is the whole political motivation and regulation to create the need for higher-efficiency vehicles. Whether that happens through global warming, or security issues around oil, or the cost of oil or consumer demand, I am not sure which of those will drive that market first, or a combination of them will drive the market like it's done for the existing hybrid vehicles.
"You need to be at a point where efficiency has reached the level where it's okay to add another thousand dollars to the vehicle cost to get 10 or 20 percent better fuel economy".
Lapp finds it curious that it appears no one else has pursued this particular vein of research. He said he's not interested in doing any patent searches nor does he plan to patent his work.
"I really just wanted to do it since we sometimes think we're at the end of the efficiency rope and we're just not. There's all sorts of neat stuff that can still be done. That's really what I was trying to demonstrate".
For Steve Lapp to go beyond where his system is at the moment would require significant outside funding that would first focus on better integrating the power electronics into the Prius Hybrid Synergy Drive.
"I've been trying to develop some corporate funding in Canada", he told EV World. He has a promise, at this point, of two additional Priuses if he can pull together the additional CAD$250,000-300,000 the project would require to properly engineer the power electronics.
Steve can be contacted at SLapp@ati.sl.on.ca.