Blowing Hot and Cold

Brian Randall, Test and Validation Manager, December 20, 2006

Editor's note: Our last blog, Good Vibrations, chronicled some of the safety and durability tests we've been running on the Tesla Roadster. It prompted a number of questions, particularly related to climate control. So we convinced Brian, who is fully booked during regular business hours overseeing the tests, to postpone his holiday shopping a little longer and spend some time after work explaining in more detail how we ensure satisfying climate control in the Tesla Roadster. Here are his thoughts…

Don't forget your coat - we test demist
and defrost in a special cold room

By throwing out the internal combustion engine and replacing it with a battery and an electric motor, Tesla Motors has achieved an amazing blend of performance and environmental friendliness. The trouble is, without an engine we end up having to rethink traditional climate control. This blog is an attempt to describe what automotive engineers refer to as the HVAC system (a crazy shorthand for Heating, Ventilation, and Air Conditioning).

With a conventional car, cabin heating comes from the engine coolant. Because the average gas or diesel engine is so inefficient, about 30 percent of the heat generated during combustion is transferred to the engine coolant, giving a ready source of heat to warm the cabin.

Incidentally, you also lose energy down the exhaust pipe and into the oil, as well as through a number of additional avenues. So you end up with only about 30 percent useful "go forward" from that expensive gallon of gasoline. This number is an estimate — it depends on type of engine and how you drive the car, but it illustrates to some extent why the Tesla Roadster is more efficient.

Now back to the original thread — waste heat from an engine can be used to keep you warm. To keep you cool in the summer months, your car has air-conditioning fitted. The air-conditioning compressor is driven by the engine. So, what does Tesla Roadster do now that we’ve junked the smelly, oily bits?

We go electric, of course! That is much easier to write than to do, however. So I’ll attempt to explain how we do this, and what tests we’re performing to ensure that our drivers and passengers are as comfortable as we can make them.

First, the heating. This is relatively straightforward. We replace the heater matrix, which would have had engine coolant running through it, with an electric heater that has 400 volts running through it. The clever bit of the design is to ensure that we have a safe system that also minimises the drain on the car’s battery pack (or Energy Storage System (ESS), as we call it).

So we use what’s called a Positive Temperature Coefficient (PTC) heater. It’s basically a resistor that increases its resistance as it heats up, thus limiting the current it can draw. That way it will never get too hot. Why do we use 400 volts for the heater? Well, unlike every other car, we’ve got 400 volts available, so we might as well use that — it means considerably reduced currents along the cables that run from the ESS at the back of the car to the heater at the front. And with the heater capable of pumping out 4 kilowatts, that should keep the cabin nice and toasty.

A Cool Primer

A car’s air-con, as we like to call it in the U.K., works like a domestic fridge. The car’s cabin is like the inside of the fridge, there’s a pump (or "compressor") to circulate the refrigerant, and an evaporator to get rid of the unwanted heat. (That's why the back of a fridge is warm - it’s the heat extracted from your milk, eggs, and beer!)

At the other end of the scale, we have fitted our cars with an all-electric air conditioning system to keep things cool. This uses a compressor similar to the one in a domestic fridge — only ours is blue and works off 400 volts. It’s at the front of the car, and pumps the refrigerant through the chiller unit in much the same way as a standard car’s system.

There is one added complication, though — we also have a requirement to keep the ESS cool. The ESS has its own coolant circuit, with a pump that circulates a water/glycol antifreeze mix round the ESS and then through a separate chiller unit to the right of the ESS behind the passenger door. This allows us to achieve cell temperatures within a range that supports long life and efficient performance. (See our recent blog, A Bit About Batteries.) The cooling will also distribute heat within the pack to minimize temperature variations amongst the cells in the system.

As well as controlling ESS temperature during any driving, we also need to keep its temperature within limits whilst charging. The ESS can safely drive the car at ambient temperatures down to -20°C (-4F°). However, charging must only take place at temperatures above 0°C (32°F). For this, we provide a heater within the ESS coolant circuit. This will only ever operate when the car is plugged in for charging.

To ensure that all this complex heating/cooling system works properly and reliably, we’re currently going through a detailed development and validation exercise on one of our prototype cars. We are doing some of this in a Climatic Wind Tunnel (CWT), a chamber that is capable of providing air temperatures in a range from -20°C to +40°C (-4F° to 104°F). This air can be blown over the car while it is driving on a set of rollers (a "chassis dynamometer"). We can thus test the car in realistic (but fully controlled) conditions.

These are some of the things we are studying:

  • Demist and defrost of the front windscreen (there is legislation that applies to this — we have to meet certain criteria to pass). This is done in controlled freezing/misting conditions in an environmental chamber. We then monitor windshield and side window clearance patterns against time. One advantage that our car has is not having to wait for an engine to warm up before we can have heating — turn it on and there it is!
  • Interior airflow distribution — this involves measuring the airflow from each cabin vent, and making sure that the HVAC controls provide the right distribution of air and the range of control that we expect.
  • Air conditioning performance — we do what are called "temperature pull down tests." We heat soak the car at 35°C (95°F), then switch on the air conditioning and watch (and measure) a wide range of underbonnet and cabin temperatures against time. We can also provide simulated solar loading during this test to pretend we’re in the Californian desert (with a bank of halogen lights).
  • Heater performance — this is similar to the air conditioning test, only this time we cool the car to -20°C (-4°F). We don’t provide snow, though!

All this is aimed at ensuring that Tesla Roadster drivers and passengers can have their fantastic driving experience in complete and controllable comfort.