The RV of the Future

I live in a motor home. Once you discover that 75.3% of the stuff stored in your garage is never going to be used, more than half the clothes in your closet don't fit and that the few useful things you have stored only mean work you are not really serious about under taking, it is easy to down size to a motor home. Everyone's idea of the perfect motor home is not the same.

Determining the perfect motor home boils down to a cost versus inconvenience ratio. The cheaper you are the more inconvenience you can accept. I am a middle of the road personality with a cheaper than normal reality. So I am going to describe the logic I use determining my concept of the RV of the Future.

A little background first. Motor homes are generally big. A 30 to 36 foot long RV is not uncommon, like a fairly large bus. Unlike a bus, RV generally do not get driven very often or very far over their life. So the power plant for motoring along the road, a large diesel or gas engine is pretty much a waste. Since the drive motor is very large, a smaller generator motor is used since it is more efficient for producing electrical power than the monstrous main engine. Hot water can be furnished by either the main motor or the generator motor, but generally a separate propane hot water heater is used because keeping the fuel powered motors running is expensive both in fuel and in maintenance costs. Combining all these functions would seem to be efficient, but how? Fuel cells are how!

That big motor in the average motor home may produce 200 horsepower peak of their rated 250 or more. Rarely is half of that horsepower ever used. So let's see what a fuel cell can do for us.

About 50 Kw (68 HP) is all that is required for normal driving. Thanks to drive train losses with converting the gas power horse power to RV motion, 100 KW will push the RV along about as well as 200 horsepower the main motor can produce. The fuel cells produce electrical and heat energy so they should be sized for needs of the campground conditions. Ballard Power manufacturers fuel cells and was nice enough to send me some information on their products. One of their most popular products is used in materials handling, fork lifts. The FC Velocity 9SSL I would use provides 20KW with an average life of 10,000 hours for $10,000 dollars. I could special order a FC Velocity designed for 25 KW, but let me see if I can make the off the rack fuel cell work.

Two 20Kw fuels cells would yield 40Kw or 54 Horsepower. That is enough for basic boring driving, but not quite enough for passing or quick acceleration. I am use to fellow travelers flipping me the bird, but some people may not like that. So I should have a little boost potential to avoid single finger salutes. Batteries? They can provide that short term boost and be useful for other things. So I will keep the three battery bank that my RV already has for back up power and add a couple. That solves my flat lander situation but not the mountain climbing mode, or does it?

Regenerative braking, using braking power to charge batteries is pretty common. A great deal of the battery boost used would be recovered in slowing the big beast down. So it is possible that 40 KW from fuels cells and some amount from batteries will cover all the traveling needs. That doesn't sound like much for a big motor home, actually, now it is a "Motor Coach". This RV of the future needs a classier name.

Comparing a fuel cell with electric drive to a internal combustion engine (ICE) is a little complicated. ICE has a narrow power band and efficiency range. A diesel that may be 45% efficiency at some point in its operating range will only average about 37% in normal operation. Electric motors average over 90% efficiency. Electric motors also have a flatter torque curve. In wheel or hub DC electric motors would be the perfect option for the Future RV. Ford, sticking with the North American theme, produced a prototype F-150 with Hi-Pa in wheel motors. The Hi-Pa Drive HPD-30 is rated for 350 NM (258 Ft-Lb) torque, 40 KW, with maximum speed of 2000 RPM. One HPD-30 paired with a FCV 9SSL (20KW) and battery bank boost of up to 20KW would be the main drive for the RV. This total of 40KW (54 HP) with 40 KW (54 HP) battery boost for a total of 108 HP (516 Ft-Lb torque) doesn't sound like much. 108 HP with 516 Ft-Lb torque though is a good match for the nominal torque provided by the Duramax Diesel 6.6L Turbo while requiring half the horsepower. The class C or bus style RV's have the aerodynamics of a brick. A modified Class A design will cut the air better improving driving efficiency. Battery load would limit range requiring adaption of driving habits (limit cruising speed to 55 to 60 mph)so this should be considered a minimum configuration. The reason that this can even be considered a minimum is that the weight of an average RV is deceptive. While the chassis of a 30 to 34 foot RV may be rated for 26,000 pounds gross weight actual weight is about 1/2 that and most of that is the engine, drive train and chassis.

Thinking in terms of torque makes understanding the in wheel Hi-Pa drive easier for gear heads. Torque is what moves the RV. Horsepower/engine speed has to be converted by gear ratios into torque at the wheels to move the RV. Once the RV is moving, gearing can reduce the torque at the drive wheels to produce high wheel RPM and vehicle speed. So if the electric motor system can provide the required torque where it is needed, forget the horsepower. Once the vehicle is to the desired speed, the motors only have to counter friction and wind resistance to maintain speed.

Accelerating the heavy RV requires a little electrical trickery. Electric motors at low voltage draw a lot more amperage than at nominal operating voltage. We want the torque not wasted heat, so pulsing DC, at rated or higher voltage for short periods or pulses (pulse width modulation), is provided instead of just varying the voltage. This pulse width modulation is in essence our gearing. By watching the gross vehicle weight and aerodynamics, the 108 peak "horsepower" is enough for normal operation. Higher performance for towing or just impressing your friends will cost more money as in more fuel cells and four wheel drive or a hybrid engine for peak performance.

Gross vehicle weight using the FCV 9SSL is interesting. The pair of fuel cells weigh 34 Kg (74 Lbs) dry weight versus Duramax engine and transmission weight of approximately 1000 Lbs. The Hi-Pa drives weight, while not negligible, is rotating which has less impact on gross vehicle weight. Subtract the approximately 600 pounds for a standard generator and the remainder of the drive train and you have a net weight savings of about 1800 pounds. Since hydrogen has a lower power to weight ratio than gas or diesel, we save on fuel weight as well. The battery weight for the FCRV will be slightly more than the normal RV battery weight by about 100 pounds (5-8d batteries versus 4). However, selecting a heavier more powerful battery set is not a bad idea.

While lithium ion battery technology gets all the press, lead acid specialty batteries have some advantages. The Surrette 4-KS-25PS battery is a solid four volt deep cycle with 1350 amp hours (versus 1250 amp hours for a five 8D battery bank). A set of three to provide 12 volts weighs 945 pounds, but has a ten year warranty. Note: both would allow for approximately 20 minutes of peak power. That would add back about 400 pounds of weight savings but with more stored power and much better longevity. While other voltage options would be better, current DC accessories rated for 12 volts provide an overall cost reduction for the RV.

Hydrogen production and storage is a big consideration. Quantum manufactures composite hydrogen storage tanks. Cost and longevity wise they are not up to snuff today, but may be on the right track for the future. A less expensive alternative is HDPE lined aluminum tanks. HDPE is high density polyethylene or plastic used to line hydrogen storage tanks to reduce leakage. The aluminum, HDPE tanks limit the maximum storage pressure to 3000 PSI. With a RV chassis, there is plenty of center line space for 50 Kg for hydrogen storage even at the lower pressure.

Producing your own hydrogen is not only a cost saver, it is now a requirement since the hydrogen highway is no where near completed. A reverse cycle fuel cell electrolyzer would be trick, but right now we have to settle for a separate electrolyzer for home refueling. There are a variety of manufacturers providing 3 to 5 Kg per day systems for about $2300 US. A solar Photovoltaic array on the roof of the RV costs about $1 a watt using Nanosolar's panels. A 25 foot by 8 foot (approximately 18 meters squared)array would produce approximately 2 kw per hour during daylight hours (10kw per day average). With current fuel prices, these systems pay for themselves in a year or two depending our your RV use habits.

For this system to be self sustaining, energy saving appliances would be needed. Air conditioning is the main power hog. Insulation of RV's is mediocre at best. Insulated window coverings and 1/2" closed cell polyisocyanurate foam sheathing would more than double the R value of the typical RV. This would reduce the typical cooling capacity required from 13,000 BTU to approximately 8,000 BTU. AT 8,000 BTU, approximately 900 watts with a duty cycle of 50%, air conditioning would require 4.5 KW per day in cooling season. A refrigerator/freezer with ice maker is approximately 1KW per day with lighting and electronics averaging less than 1 KW per day. Properly designed, during the worst energy usage season, an extra 3.5 Kw per day would available for hydrogen electrolysis and battery trickle charging. A second AC could be added for peak cooling demand which would eat into our hydrogen budget a bit. Fortunately, that worst energy usage season would also be the highest solar energy production season. We can fine tune these estimates, but they seem quite reasonable at this time.

So how does all this compare cost wise? Two FCV 9SSl totaling $20,000 is the major cost. It is hard to find a real price for a Duramax with transmission. My estimate for brand new out of the crate would be $15,000. Differences between gas and diesel for new RV's range between $15K and $30K. Some of that is due to chassis type,but as and estimate, FC power will cost about $5,000 more diesel and about $10,000 than gas. For another $10,000 you could add an extra fuel cell to impress your friends. I could not get a good price estimate for the HPD-30 in wheel motors. An addition $7,500 should cover two HPD-30's and drive/regenerative braking modules. Fuel cost for most RV's is nearly impossible to figure. The average miles put on a RV is only 20,000 miles with about 12 miles per gallon average. At $3.00 per gallon that is only $5,000 dollars. On the other hand electrical and propane cost is about $120 per month average. Over five years that is another $7,200. For a wilderness camper, the saving would be much more because generator electricity is more expensive.

Maintenance wise, the Ballard fuel cells are rated for 10,000 hours. That is virtually maintenance free. Then the PEM, Proton Exchange Membrane, should be replaced if performance is degraded. If the Fuel cells are installed to allow easy access, replacement could be a simple maintenance program every 4 to 5 years. The electrolysis refuel system would have a similar PEM schedule and filters/water treatment system maintenance schedules.

Depending on your expected use, the FCRV can be a good option now. For example, FEMA responders often take their RV for housing and office space in areas with little chance of reliable on site electric service. The totally self contained FCRV would be well worth the added cost.

I need to better model the average energy per mile driving to allow for regenerative braking and wind loading, but my estimates should not be far from reality, provided the true gross weight of the RV is 50% of the gross chassis rated weight. Ballard Power does make a 75KW FC 900 series for automotive that is not available for this article. While a bus style 150kw FC would seem to be the proper selection for the RV, real world conditions should not require that much continuous power. I will proof and modify this in the future, for now though it may be of interest.

So how slow will this bus be? Acceleration A = T(orque)/M(ass)x R (tire radius)

T = 700 M=3000 and R= 1 for simplicity So acceleration is equal to 0.23 meters per sec. That is slow! We need to get to 100 KPH(62 MPH) in a reasonable time, say 20 seconds, which is an acceleration of 5 meters per second. Back calculation then requires a torque at the large wheels of 5 MPS times 3000 Kg times 1 meter = 15,000 NM So changing the radius of the tire to something more realistic, 0.25 meters, might work but we have to look at the top speed which is limited by the 2000 RPM of the in wheel motor. If we feel we can get away with 120 KPH which is a top speed of 75 MPH, we get a wheel circumference of 1 meter. That gives a radius of 0.16 meters or a tire that is 0.32 meters tall or 12.5". That is a pretty unrealistic tire. So we have to compromise to something realistic, but smaller than we would normally wish to reduce the torque. Let's pick a 20" tire. That has a radius of 10" or 0.254 meters.

Torque required with this wheel radius is 3810 NM. That would mean the Hi-Pa HPD30 is not a good choice. We need 1905 NM per wheel for two wheel or 950 NM per wheel for 4 wheel drive. Unfortunately, it is hard to get in wheel motor information, and most designs are for light passenger vehicles. The HPD series has the higher torque HPD35 and HPD40, but according to the literature, the KW per torque is less attractive than the HPD30. Solution, tandem rear wheels and front wheel drive. That would give us 6 time 350 or 2100 NM total torque. Not as much as we want, but what will it do?

Still shooting for the 5 MPS acceleration we have 2100/(5*.254) equals 1650 kilograms or 3630 pounds of vehicle. That is a little small for a "motorcoach". We only need about 60 seconds of extra acceleration, so what low weight options are there?

Azure Dynamic makes an AC motor for medium weight trucks. This option brings us back to typical mechanical gearing. Also the AC90 manufacturer states that regenerative braking options are there. Weight and input power wise it is an interesting option. The AC90 with controller weigh in at 216.5 Kg (476 pounds) with 97 Kw peak shaft power and 50 Kw continuous power. While the torque is about the same as a pair of HPD30 in wheel motors, we can more easily use mechanical gearing to get the wheel torque required for acceleration. Shame really, because the in wheel design could do the same thing. I have an email out to the HPD30 manufacturer to see if they have addressed that issue.

This change brings me right back to my estimated 50Kw which we have only 40Kw to work with plus battery boost power of up to 40Kw for a limited time. The power curve for the AC90 gives us an operating point at ~4200 rpm where we meet the 40 Kw available from the pair of FC Velocity (SSL). It is a compromise that will work, maintain our energy budget, provide reasonable acceleration and thanks to mechanical gearing, maintain a reasonable cruising speed. If I were King, I would get Azure Dynamics to rework the AC90 for 40Kw continuous power input with 80Kw peak. On the other hand, I might get PML, the manufacturer of the HPD30 to modify their design. One might think that I would get Ballard Power to custom build a 25Kw version of their FC Velocity 9SSL, but with mechanical gearing being required anyway, there is no need, 40Kw can push the "motorcoach" along nicely once we accelerate the load.

So my dream RV of the Future may be built with off the rack components right now. Using a pair of FC Velocity 9SSL or three if I wanted more power, only costs $10,000 per fuel cell. A 150Kw fuel cell for a bus costs upward of $600,000 US. So hydrogen fuel cell vehicles are not as much of a dream as you might think. While I may come back to this post to fine tune the basic design, I will tackle either a zero emission fishing boat or refine my green Tahoe next.