Toyota to add solar panels to Prius hybrid: Nikkei

The issue here could simply be semantics, and/or mis-translation between a Japanese news source and English. "Cooling the air" (by drawing outside air into the car with the blower) in Japanese might have been translated to "air conditioning" in English. Easy mistake to make. Almost requires a translator not only fluent in both languages, but also scientifically savvy -- probably a rarity.

Many smart engineers have posted on all the blogs that the most power you'll get from PV cells on the roof of a Prius is about 250watts peak (12:00pm, sunny day, southern US latitude). I've read in various places that the Prius A/C compressor is variable speed, requiring between 1500 and 5000 watts. So, PV cells on the roof, at best, could provide only 16% of the compressor's power consumption. Not gonna happen, my friends. Unless Toyota has a secret earth-shattering technological breakthrough, it is most likely that they are designing a system to have roof-mounted PV cells integrated with the standard blower.

btw - all the engineering calculations I've seen involve current PV cells technology capable of 12-20% energy conversion efficiency. In laboratory studies, the highest efficiency achieved is about 40% using multijunction concentrator PV cells. So, even with those exotic materials, we're talking about 500-750 watts peak power available, still well below that required to run the A/C compressor. Anyone know the cost of the latest exotic PV materials? My hunch is an outrageous cost.

 

Great phrase! What an awesome way to put it!


All cars need an MFD to rub the mpg in our faces! I truly believe it affects driving habits for the better. I was recently on a business trip in Honolulu, and had to rent a "regular car". Lo and behold, I often found myself reverting to the typical southern-california highway speeds, which I learned you don't want to do in Hawaii (extremely laid back culture, no one in a hurry). In my Prius, I drive to maximize my mpg, within reason not to piss off other drivers. However, in a rental, I found myself with a lead foot (old habit). It cost me dearly in my first speeding ticket in 10 years. I likely would have driven slower and avoided a ticket if I had an MFD rubbing the mpg in my face!

 

Well, it *is* all downhill. *Going* to work anyway. Coming home ... hmmm.

 

5000 watts seems very high. Over on GreenHybrid, I saw an post which claimed the low-end of consumption was 300 watts. My 15-year old 3.5 ton 9 SEER Carrier heat pump condenser (which was replaced several years ago with a more efficient unit) only consumed 4500 watts when it was running flat out. That's enough cooling to keep an 1800 square foot home cool in the hottest desert climates. And RV rooftop air conditioners typically consume around 1700 watts.

 

If I am interpreting this presentation correctly, max power is 3.4 kW for the current Prius AC unit. (see page 9)

 

It certainly looks that way. I thought that sounded way too high compared to a home unit, but it could very well be. I suppose the thermal load in a car is much higher due to the large amount of glass area. It also makes sense from a back of the envelope standpoint. The HV battery is ~1.3kWh, but only ~500Wh is useable. An 8 bar (full green ~80% SOC) battery being discharged at 3.5kW to no bars (~40% SOC) would last about 8.5 minutes. Thats probably longer than I've ever been able to sit with the AC on high before the ICE kicks in, but then again its usually not spanning the full range of SOC either and the AC is not the only power draw on the battery but it should certainly be the dominant one.

Rob

 

No, cooling performance would refer to the amount of heat the A/C can remove. One ton of cooling equals 12000 BTU/hr, or 3.5kw. So 3.4kW of cooling is a tiny bit less than one ton, or 12000 BTU/hr of cooling, roughly the same as a medium size window A/C (which consumes about 1kW to do so).

 

I totally agree. I really enjoy trying to drive the Prius as well as I can. When I get back behind the wheel of my WRX it doesn't take long for my old habits to creep back in. Need to sell the WRX and buy another Prius.

Rob

 

You might be right, but I interpret it differently. The info sheet uses a term "cooling performance". I think they are referring to the refrigeration capacity, which is more typically given in units of BTU or "tons". The conversion is kW x 3414 = Btu. 5000 watts does seem high, but maybe that number refers to the power surge required to start the compressor. Compressors are notorious for drawing very high starting currents. The other reason I think 5000 watts might be right is I once saw a tech document stating the Camry A/C compressor requires up to a little more than 6000 watts.

Between numerous media reports and hybrid vehicle bloggers, I've seen all sorts of differing numbers. I wish someone out there could set the record straight once and for all -- provide substantiated numbers for rooftop solar capacity and A/C requirements. I'm just a nuclear engineer, not an automotive engineer, so any numbers that I quote are just repeated from elsewhere, numbers that pass my litmus test.

 

Sounds reasonable to me, because when "gliding" (no charging) with AC on at 95 degrees, the SOC goes from 6 bars down to 4 bars after 2 or 3 long glides, and then it disables its glide mode.

 

This was discussed on PTS awhile back. I seem to recall 1.5kW being thrown out, but couldn't find that post again. Ken@Japan posted the PDF that I posted above.

 

The PDF that you posted shows 1.5 kW consumption at 95 deg F (35 deg C) with recirculation.

 

Actually, it shows 1.5 (no units). I guess we can assume kW.
Also, in the upper right it says the vehicle model is an SUV.

Not an impressive document. Many discrepancies, e.g., units missing on many charts. From what I've read elsewhere (and intuition, if that's worth anything), I believe the argument that electric-driven A/C is more efficient than engine-driven. However, this document makes it hard to grasp. Slide 6 doesn't help any. In fact, the chart makes it appear that electric-driven A/C is less efficient. Something's just not right about that chart, but I can't put my finger on it.

 

I agree and have an additional reason.

Batteries and electronics hate heat. Batts are less efficient and electronics will break down if too hot. The AC (or solar powered ventilation) not only improves human comfort, but improves battery efficiency (?) and reliability of batts and inverter etc.

 

There has been a lot of discussion in this thread questioning just how much a small PV panel could receive per day in terms of solar energy.

The cells on this converted Prius take up about 1.3 m2 of roofspace:



The NREL figures for solar energy insolation in the United States show that, on AVERAGE, the daily solar energy falling on 1 m2 of panel is 5.5 kWh. Of course you would get more than this if you live in the Southern states and less in the north. Likewise, you would get more than 5.5 kWh per day in the summer, and less in winter.

So, parked outside, a 1.3 m2 Prius rooftop array could receive on average 7.2 kWh of energy per day over the year.

Today's best commercially available solar cells (SunPower A300) convert 21% of that into electricity, giving you 1.5 kWh of electricity on average per day. That's enough for 7.5 miles per day of EV range, or quite a lot of AC cooling.

Finally, remember that there is no reason why you couldn't press a button on parking to allow an extending sheet of solar fabric (like nanosolar's material) to cover the entire car, thereby keeping it cool anyway, and as a bonus presenting a 7 m2 panel to the sun to collect 8 kWh per day (40 miles EV). With Nanosolar's cells now selling at $0.99 per watt, it won't be long before this type of thing is a no-brainer.

The future is bright for solar.

 

Very nice analysis. Some things to correct, though. The data you quote is for a solar collector plate facing south, tilted for latitude.
http://www.nrel.gov/gis/images/us_pv_annual_may2004.jpg

Also, the data is geographically weighted (average by land area). If you weight the data by population (highest densities on east coast and rust belt), I would visually interpret the average population density-weighted solar insolance to be between 4.5 and 5.0 kwh/m2/day.

So, for a no-tilt car roof, you need to factor in the cosine of the average latitude, 37.7 degrees North. Mean center of United States population - Wikipedia, the free encyclopedia

That results in a range of 3.5-4.0 kwh/m2/day. Let's continue using the middle of that range, 3.75 kwh/m2/day.

3.75kwh/m2/day * 1.3m2 * 21 % efficiency = 1.02 kwh/day energy from the roof.
Assuming an average annual daylight of 12 hours/day, 4.875 kwh/day / 12 hrs/day = 85 watts average power from roof.

This is lower than your figure, and lower than what I've seen elsewhere (100 watts average, 250 watts PEAK). And, it's still woefully inadequate to power the Prius AC compressor continuously during daylight.

Obviously, you don't need to run the AC continuously while the car is sitting in a parking lot all day. You could pre-program the AC to come on at a certain time, let's say 10 minutes before you need to return to your car, or you could start the AC with a remote radio signal (would have to be more powerful than the standard keyfob). Someone who does a lot of mini-trips during the day may want this a few times a day. My hunch is that 1.02 kwh/day is probably enough to supply this kind of demand, but you need to be able to store it in the battery (and suffer a little bit of efficiency loss there). And also, those that need the AC more will likely benefit from more solar energy available.

So, as they say in "mythbusters", the ability for rooftop PV cells to power the AC is "plausible", but you need energy exchange with the battery and you could only run it for a small fraction of the day.
I think it would make more energy-sense to just use it to power the blower. You don't need to knock the inside temperature down from 150 degrees to 70 degrees. Knocking it down to the the outdoor ambient temperature is adequate for most climates. Something is better than nothing.

Lot's of variables and assumptions. If the original news story out of Japan is true, I'm sure there are some Toyota engineers busy figuring this all out. I'll bet they've already retrofitted a Prius to experiment with this concept and obtain some empirical data.

 

If it just powers a ventilation fan, I hope it won't be the same fan as when we are driving... the fan would be used 10-20 times more than in the normal usage and it would start to become noisy quite rapidly.

 

I haven't seen any news article that mentions using the solar panels to power a cooling fan while parked. That was just some PC poster's postulate.

 

Ok, 3.5kW cooling power = 12,000 btu = 1 ton makes a lot more sense. At an EER rating of 10 that would draw about 1.2kW. According to the graphic below, discharging from a more typical 6 bars to 2 bars is only 15% SOC, or about 196Wh. At 1.2kW draw, your AC should run for about 10 minutes before cycling on the ICE if you started at the top of bar 6. Realistically it would be less as we know the car draws about 300W, and the fan on high about 180W. With all that it would be about 7 minutes. Sounds plausible. The AC load will also go up quickly if the EER is less than 10. An EER of 8 would give the rumored 1.5kW, and would put the total load at ~2kW and the run time at about 5-6 minutes.
Image:Index.54.jpg - EAA-PHEV

Figuring the daily kWh from the daily radiation per m^2 is pretty clever. 1.3 m^2 for the roof of the Prius is a very handy number. The problem with trying to back that into an instantaneous power is you have to make an assumption on daylight hours. The 12 hours used above is probably way to much, in terms of "peak sun hours" or the hours during which a solar array is productive. I think somewhere in the 5-6 peak sun hours range is more typical, which would double the 85W estimate to ~170W. It probably makes more sense to just look at the actual DC rating of a panel using these cells though. For example, the one linked below is 210W peak rated for 1.244 m^2 area. Pretty close to what you should be able to do on the Prius. Also bear in mind the cumulative effect is only really relevant if you have somewhere to store that energy.
SunPower 210 Watt Solar Module - SPR210 [SPR-210] - AU$1,695.00 : Renewable Energy Store, buy alternative and renewable energy equipment - solar and wind power

You will take a hit for sun angle of course but as I was kind of saying before, the higher and more direct the sun the more you really need the extra cooling. In the middle of the day in midsummer in Phoenix the sun is at 70-80 degrees. Sure in winter its down to 35 degrees, but then its 65F outside and you don't need much AC I like the fact that you get the most power out when you need it most. To look at it more quantitatively, for the month of June a 1kW rated array at 90% efficiency will make 187kWh flat, but only 167kWh at the "ideal" angle of 33 degrees. The "ideal" angle is really a compromise between the best performance in winter and summer. Since in a car you only really need AC in summer, a flat angle is not such a bad thing. Scaling the above number for a 210W panel, you should be able to collect 39.27kWh, or about 1.3 kWh per day in the summer in Phoenix.

Since the solar will have to be tied into the HV system anyway in order to run the AC, it would be cool if they did use it to top up the HV battery while the car sat. Coming out to a full battery at a reasonable temperature and with reduced AC load due to the cooler cabin temperature would make a pretty big difference in my summer numbers for sure. Might finally give us a reason to use EV mode.

Some handy utilities:

Residential solar output calculator:
PVWATTS v. 1

Sun Path Viewer:
sustainable by design :: sol path

Rob

 

Hence my usage of the word "if"