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Peltier Intercooler Water Spray

Spraying COLD water on your intercooler- well, that was the idea

by Julian Edgar

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This article was first published in 2004.

In this introduction we’ll do what journalists never do – well, magazine journalists anyway. And what’s that? We’ll tell you the punchline before we even begin. The Peltier-based intercooler water spray covered in this story was unsuccessful – it didn’t work. Well, not adequately anyway. So why tell the story? Two reasons: firstly, why it didn’t work is very interesting, and secondly, it’s also very good background for those who wonder about using Peltiers for intercooling purposes.

It frequently crops up in web discussion groups: why can’t Peltier coolers be used to build an intercooler? It seems reasonable enough: Peltier coolers are designed to work off car-type voltages, in recent times their prices have been dropping fast, and they do what you want an intercooler to do – remove heat. (See the breakout for more on these fascinating devices.)

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But the short answer regarding Peltier intercoolers is that with the current level of Peltier technology, it’s not going to happen. A powerful Peltier device like this one is rated at 80 watts of electrical power. Wile the relationship between thermal heat movement and electrical input power is not 1:1,  the power rating of a turbo car intercooler (eg the B4 Subaru at 13.4 kilowatts) is so much higher that you’d need something like 15 Peltiers to do the same job – and that’s ignoring the very real problems of the actual heat exchange process and the power consumption from the battery. It’s certainly possible that one day intercooling will head in this direction, but so far it’s not been viable.

That’s intercooling – but what about cooling just the water that you spray onto the intercooler core? There’re a few efficiency loss steps along the way (the Peltier has to act through a heatsink to cool the water which in turn has to act through the intercooler metalwork to cool the intake air) but taking this approach has a large advantage.

The advantage is this: water can be cooled over a reasonably long period (eg 5-10 minutes) and this ‘assembled coolness’ then dumped in one hit. This is potentially very effective because quite often a turbo road car needs a big power squirt – but then nothing much more for some time. An overtaking move, or a quick traffic light sprint, as examples.

Peltier Coolers?

Sometimes called ThermoElectric Coolers (TEC), Peltier modules consist of a number of p- and n-type pairs (couples) connected electrically in series and sandwiched between two ceramic plates. When connected to a DC power source, current causes heat to move from one side of the TEC to the other. This creates a hot side and a cold side on the TEC. A typical application exposes the cold side of the TEC to the object or substance to be cooled and the hot side to a heatsink which dissipates the heat to the environment. A heat exchanger with forced air or liquid may be required. (As clever as TECs are, they can't eat heat - only move it!) - www.melcor.com/faq.htm

Peltier module manufacturer Melcor has plenty more interesting and detailed design information on their site: www.melcor.com

The First Design

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I decided to put the theory to the test and bought two 80W Peltier devices from www.oatleyelectronics.com. They cost AUD$18.50 each. In a typical application (eg a portable car fridge) the Peltier device is sandwiched between two heatsinks, often with a block of aluminium in there as well to give some clearance to the walls of the fridge. The inner heatsink transfers the cold to the interior of the fridge (or more accurately, removes heat from the interior) while the outside heatsink gets very hot and is cooled by a fan. (Here the inner heatsink is shown – smeared with heat sink compound to better aid heat transfer.)

So a Peltier device acts as the conduit, moving heat from one face to the other.

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The design was for an interim water storage device that could be plumbed into the intercooler spray line between the pump (attached to the main reservoir) and the nozzle. The 250ml of water held in this ‘cooling container’ would be directly cooled by the Peltier device. When the spray started, the water in the container would be pressured by the pump and would flow out through the nozzle, being replaced by more water coming from the reservoir. Of course if the spray ran continuously, the ‘cool’ water would soon be used up, but in typical discontinuous use I figured the cold spray would be good for power bursts.

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I bought a high quality plastic box and two heatsinks, one a little smaller than the other. The smaller heat sink mounted inside the box lid, its fins projecting down into the box volume. In the lid of the box I cut two 40mm square holes, in which two Peltier coolers sat, their lower surfaces in contact with the inner heatsink. On top of the box - in contact with the upper surface of the Peltiers - was an aluminium bar, while the second (larger) heatsink sat on the strip. (The bar gave enough clearance for the Peltier power supply cables to exit under the heatsink.) The sandwich assembly was bolted together with two through-screws and nuts and all contacting surfaces were smeared with heatsink silicone grease.

To see how effective the system would be, I filled the box with water (it took about 250ml) and then connected the Peltiers to a bench power supply. A small but powerful fan was aimed at the upper heatsink.

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Within a minute or so of power being applied, the upper heatsink was hot to touch – heat was being drawn out of the water and dissipated through the heatsink. However, even after 5 minutes, the water inside the box had barely changed in temp. I then measured the actual voltage that the nominally 12V power supply was providing and found that under the substantial 160W load it had dropped to around 9V. Concerned that the lower voltage might massively decrease the Peltier performance I then moved the test assembly to my car and ran it straight off the battery, engine running.

This time the upper heatsink got even hotter, but the water inside the box wasn’t getting any cooler. It was always going to take a certain amount of time before the water became adequately cold, and I figured that 10-15 minutes was about the realistic maximum this could take. But after 10 minutes the water was – if anything – only a few degrees cooler than at the starting point, and after 15 minutes it was much the same.

Despite the Peltiers dragging heat out of the water, not much was happening to the temp of the water.

Why Didn’t It Work?

There are a few reasons why the system didn’t work. One is the specific heat capability of water. As we have covered in the past in water/air intercooler stories, water has a massive specific heat – that is, even a small volume can absorb a huge amount of heat with very little temperature increase. (That’s why when you put a saucepan of water on a stove, it still takes quite a long time to bring the water to boil – even though you might be pouring in a few kilowatts of power.) And the same characteristic applies when trying to cool it – you can remove a lot of heat without the temp dropping too much.

But the most important point relates to getting rid of the heat. The external heatsink has to shed all of the heat that is being drawn from the water, and as we said above, that’s a LOTof heat.  Despite the use of a larger heatsink on the ‘air’ side of the cooler, the ability of air to take that heat is much less than that of water, so in this case the heatsink was working overtime... all of the time.

Simply, even with a relatively large external heatsink, not enough heat was being able to be shed to cool the water. But what about using an even larger external heatsink? As an experiment I bolted a huge 550 x 140 x 13mm alloy plate straight to the Peltiers. That’s a helluva hefty piece of aluminium! And this time the system started to work – the water inside the plastic box cooled down rapidly to a temp of 15 degrees below ambient. Incredibly, even this size of heatsink still got hot though – it needed fins to better shed the heat.

Alternator Drain?

One point that many people will make is that the electrical load being placed on the alternator by the Peltiers (180W in this case) will subtract from the power available to drive the wheels. However, it’s easy to get around that – simply switch off the Peltiers (using a relay and a boost pressure switch) whenever the car’s on boost.

The Final System

Nothing daunted, I continued with the development process.

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One reason I was happy to continue is that I managed to source a very large aluminium heatsink. The unit, which is sized at 250 x 185 x 60mm, came from a scrap metal dealer and cost just AUD$6. This looked like it would be very effective as the external heatsink, shedding the heat drawn out of the water to the air. However, its finned-on-both-sides design meant that an aluminium angle bracket needed to be used to connect a mounting plate on which the Peltiers sat to the main body of the heatsink.

Other than that, the design was much the same as before – an internal heatsink projecting into the water with the Peltiers sandwiched between this heatsink and the main outside mounting plate, which in turn connected to the large heatsink. In addition I placed two 12V PC fans at the base of the heatsink (which was positioned vertically), so that convective airflow up past the heatsink would be encouraged. (By now this was going to be a boot-mounted design!) In this view, the lid of the plastic box – which comprises the lower half of the box – is missing.

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I filled the box with water and connected power to the Peltiers. But again the result wasn’t all that effective – the heat transmission path from the Peltiers to the large heatsink was too high in resistance, despite thick aluminium being used to connect the two. The aluminium plate on which the Peltiers sat was very hot, and the angle connecting this to the heatsink was hot. But the heatsink was only just warm to touch. It seemed that the heatsink itself had to be in direct thermal contact with the hot side of the Peltiers if it was to work well.

And without having access to a mill to shave off a whole bunch of fins, that was too hard to do.

Car Body as the Heatsink?

Quite early in the development process I considered using the whole car body as a heatsink. However, the Peltiers have to be very well connected thermally to the heatsink and there no real way of effectively doing that if the car’s body is being used as the heatsink.

Conclusion

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It is probably possible to develop an effective Peltier-based intercooler water spray. However, it would need a very large heatsink, cool a relatively small volume of water and consume quite a lot of electrical power. And there’s certainly no easy and cheap DIY approach that will work well.

An Alternative?
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At the time of writing Oatley Electronics has available a pre-made 1-litre insulated water tank with attached Peltier cooler, heatsink and fan. It works from 12V DC and costs just AUD$37. While this looks perfect for an intercooler water spray application, we would suggest that based on the size of the fan and heatsink, it is a low power device that would only cool the water adequately if run continuously (ie 24 hours a day).

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