Cabin Heat - the Enemy of EV Range?

THE FREE (HEATED) RIDE IS OVER

For the past century of internal-combustion (IC) powered automobiles, we've taken for granted the luxury and convenience of having heat (unless you owned an air-cooled Volkswagen, but that's another topic). When we've needed heat to maintain comfort (or in more severe climes, to survive), or clear the windshield of our IC cars, we've used "waste heat," excess thermal energy which is a side-effect of IC power. This excess heat is conducted away from the hot combustion regions of the IC engine and into the air around the vehicle. So using some of this heat before it's simply exhausted from the vehicle is for all practical purposes free. We incur almost no energy consumption consequences when we turn on the cabin heat in our IC car. (Actually, in extremely cold conditions, some IC vehicles can have trouble reaching a sufficiently high engine temperature for efficient combustion and oil viscosity. So in these cases, turning on cabin heat might further over-cool the engine.)

The amount of energy required to maintain human-comfortable temperatures in a poorly-insulated metal box which is constantly being cooled by 65 mph, 10 degree Fahrenheit air flowing over it is impressive. While there are parts of EVs that get warm during operation, there isn't nearly the amount of excess heat that has been available in IC vehicles. So EV manufacturers have had to resort to using some of the precious stored electricity from the vehicle's battery to make heat. The heating system in our Ford Focus Electric, and probably most EVs, is a resistive heater. A resistive heater uses electricity to heat conductors - wires - over which cabin air is drawn. The shocker is how much power the heater draws. My calculations (see below) suggest that the heater uses almost 7,000 watts of power - slightly more than a typical home electric oven.

GM's pioneering EV1 and Toyota's 2004 RAV4 EV incorporated "heat pumps" to heat and cool the interiors, but I've thus far found no evidence that any current EVs are employing that technology. Heat pumps, while power efficient, work somewhat slowly at moving heat from one place to another, and would probably be a poor choice for an environment in which the entire volume of the cabin could lose all its heat during the 30 seconds it might take to buckle the kids into their seats.

When I turn on the heater in our Focus Electric, its range estimate falls by slightly more than 30 per cent - it is assuming that I'll leave the heater on for the entire journey (which is one of many flaws of range estimation). In many cases, the heating system will be able to reach the desired temperature so that either I or the thermostat will turn off the heating element. When I tested the range impact for this article, the full-charge range estimate (which varies based upon the previous driving cycle) for the Focus at the time was 91 miles, and I estimate that the Focus would have to be traveling at 60mph on level ground in moderate temperatures to achieve that. The Focus' battery pack has a capacity of 23 kilowatt/hours (kWh). So we can extrapolate that for our 2013 Ford Focus Electric:

• 91 miles @ 60mph = 1.5 hours
• 23 kWh full battery / 1.5 hours = 15.33 kW (20.56 horsepower) @ 60 mph  
• 91 miles no heat / 63 miles with heat = 1.44 = 44% more power with heat
• 0.44 heater coefficient x 15.33 kW @ 60 mph = 6.75 kW heater power 

So our Focus Electric's heater has a devastating impact upon range. In the worst case, a new EV owner might spend the entire spring and summer season commuting 60 miles to their workplace and back, arriving home each evening with 15 surplus miles with which they could run errands before recharging. But when the winter arrived, they'd discover that in addition to some range lost to battery efficiency at low temperatures, they would be unable to complete the same journey while maintaining any sort of cabin heat. We live in Southern California, and our Focus has heated seats (I envy Chevy Volt owners' heated steering wheels). So down to the low 40s, we've made do with being a little bit cooler and cranking up the seats (which have no noticeable impact upon range). But if you lived in a really cold place, you'd have to deal with a lot of discomfort, or come up with an additional charging stop.

In our Focus Electric, which has an "automatic climate control system," selecting any temperature that's even a single degree above the current cabin temp will energize the heater, and cause the range estimate to plummet until it reaches that target temp. The same is true for the Defrost mode. So it's a bit more involved to use an EV's climate system if the intended journey approaches the ultimate range of its battery pack. The Focus Electric is what I call a "conversion" - it utilizes many parts and systems from the existing Ford Focus internal combustion car it's built alongside. The "legacy" HVAC (heating, ventilation and air-conditioning) control system operates as blithely unconcerned about power consumption as it does in IC vehicles, and every time I call for a little fresh air, the HVAC gleefully turns the fan on full blast and cranks up the heat or A/C compressor, forcing me to frantically start poking at its controls to limit its effect upon vehicle range. A vehicle purpose-designed to be an EV could and should incorporate systems and operational modes which are more energy-aware. I wish that the Focus Electric had a low-power heating element just to keep the windshield from fogging. Instead, I engage the mighty 7 kilowatt heater system and watch the battery gauge instantly plummet to 2/3 of its previous range estimate.

If you operate your EV in a region with frequent or prolonged periods of intense cold, you should consider that its maximum range could dramatically change during the cold season due to heater use. Here are some strategies to limit the range-reducing effects of using cabin heat on an EV:

• Bundle up in a lot of clothes and avoid using the cabin heat. However, it can be impossible to go without turning on the defroster. When it's cold outside and you're exhaling inside, you eventually end up with either fogging or frosting condensate on the inside of the windows. In our Focus Electric, there is little choice but to engage the power-sapping heater when the "defrost" function of the HVAC system is called. 
• Use cabin preconditioning to preheat your vehicle while still connected to a charging source. In addition to passenger comfort, this helps with interior defogging and exterior defrosting (when parked outside), but at no small cost in electrical energy from the utility company. Much of this cabin heat will be quickly lost in very cold conditions when in motion. If there are no charging facilities at the other end of the commute, preconditioning will be unavailable for the return trip. Cabin preconditioning, while not affecting battery range, does have its cost (see "Cabin Preconditioning" below).
• Use the heat sparingly. In the pursuit of range, we're willing to have a cold cabin. But I'm far less cold-averse than most people, and if it were REALLY cold, or had passengers, I'd still want some heat. We use our seat heaters in lieu of cabin heat whenever possible. We're all used to being warm and toasty in an IC car. I don't see that happening without significant consequences any time soon in an EV. 

Of course, if your journeys use only a fraction of a full battery, and you don't mind using more energy, you can crank up the heat and stay toasty warm. Even then, you'll be spending less on energy and have a lower carbon footprint than you would in an IC car.

CABIN PRECONDITIONING - A HIDDEN COST?

An oft-mentioned feature of modern electric cars is "cabin preconditioning" or "climate preconditioning."  And for the 15 years I've been hearing about it, I've found it both an appealing idea - your car is already warm in the winter, or cool in the summer - and what sounds like a perfectly logical strategy for an electric car: that you use your home's boundless electrical supply so that your precious battery charge can be preserved for propulsion.

. . . and work it does. Our Internet-connected Focus Electric provides on-board and remote (via website or smartphone app) programming of "Go Times" - the anticipated time of departure. Select one of the preset temperatures, and for the 15 minutes prior to the Go Time, the climate system attempts to reach and maintain that target temperature. Alternatively, the user can "Remote Start" the Focus Electric (a funny expression, since there really is no "starting," per se, except to put the car into Drive mode, which is decidedly NOT what you want to do when you're not in the car) from key fobs, smartphones, and the Web. If the user is prescient enough to leave the climate system in the "on" position and set to a target temperature (I'm so focused on managing power, I don't even run the climate control beyond getting barely comfortable while in the car), the remotely-started Focus will attempt to achieve that temperature for a maximum of 15 minutes.

Again using our extrapolated Focus Electric data:

• 6.75 kW heater x 15 minutes / 60 mins in an hour = 1.69 kWh per 15-minute precondition
• 23,000 watt hour battery / 91 miles = 253 wH/mi @ 60 mph
• 1.69 kWh 15-minute precondition / 253 wh/mi = 6.67 miles @ 60 mph

So a single morning's 15-minute precondition cycle would be equal to almost 7 miles worth of highway driving. Assuming you preconditioned just once each weekday morning, that's 21.67 avg weekdays/month x 1.69 kWh = 36.62 kWh/month. Using our (Los Angeles DWP, late 2013) highest tier electrical rate of about 20 cents/kWh, that's $7.32/month to precondition your cabin. That might not seem like a lot, but I drive our EV 5 miles every day of the month with that much electricity.

And that doesn't even take into consideration that you might turn on the heat while you're driving.

So cabin preconditioning is nice. There's the convenience of getting into a car with a comfy cabin and a clear windshield. But if you are interested in EVs because you want to reduce your energy footprint, just know in advance that cabin preconditioning can have significant energy-use consequences.

AIR CONDITIONING - NOT THAT BAD (SO FAR)

In the case of our Focus Electric, the energy impact of turning on the air conditioning compressor to cool the interior or de-fog the windshield appears to be far smaller than the heater. During the same test I performed with the heater, turning on the A/C dropped the Focus' range estimate from 91 miles to only 89 miles - representing only a few hundred watts of power. This is pretty impressive, given that just 30 years ago, automotive air conditioning compressors used as much horsepower as our EV does to move through the air at 55mph. We haven't yet used the vehicle in the truly hot weather that we can get here in SoCal, so we don't know how high temps will further reduce battery range, or whether the Focus' high-efficiency electric A/C compressor can refrigerate well enough to keep us comfortable in triple-digit weather. I'll report as I can.

During our first month of EV ownership before our Level 2 EVSE was installed, I plugged our Level 1 EVSE through a Kill-A-Watt. This product is an electrical energy logging device, intended to let consumers determine how much energy any given appliance in their home uses. When I compared the Kill-A-Watt's logs to the Focus Electric's on-board log of energy use, I discovered that the Kill-A-Watt reported over 30 per cent more energy use than the Focus - and this was without doing any cabin preconditioning. I have no way of knowing whether the Kill-A-Watt or Focus are accurate or not in their data logging, but the suggestion is that the car's records might not reflect the amount of electricity actually pulled from the energy grid and billed to the user. This does make sense - the car isn't responsible for whatever inefficiencies there might be in the rest of the power transmission process. But the point is that an EV's historical log of electrical use probably doesn't present the entire picture of electrical cost-of-operation. The Kill-A-Watt is 120 volt, 15 amp maximum, and can not be used with our 240V, 30A L2 EVSE. I intend to install a separate energy logging system in our home electrical system to accurately determine how much energy the EV is using - if I do so, I'll report that here.