Optimisation exercise for car aiconditioner

[juk]

I've got a lotus elise, which has a couple of problems.

1). I filled the engine bay up with a dirty big motor and supercharger, so i can't fit an aircon compressor in there.
2). The engine is in the back, and the radiator in the front, so aluminium pipes full of hot water run from front to back and return inside aluminium channels, these channels get hot, turning into radiators and blast you with heat.
3). The radiator heat soaks and so the front of the car blasts you with heat too.
4). I live in Australia, where the sun is very hot, very very hot, crazy hot and the car doesn't always have the roof on.

All this isn't so bad when you're moving, but stuck in traffic and you begin to regret taking this car.

So i thought i'd add aircon, hence why i'm here.

The current plan of attack is based around an electric compressor running 12 volts. I have a 95 Amp alternator, car will run on 45 amps including all radiator fans including the ones required for aircon, leaving me 50A to play with without having any headlights on. With headlights on, i'll have to turn the aircon down or i'll be depleting the battery.

Compressor is this model: http://www.masterflux.com/products/sierra/?pid=23 which pulls 52 amps at 12 volts. It has a PWM style controller where if you turn it down to half, you get roughly half the amps, so that's a win.

I'm going with generic evaporator about 200mm*200mm *40mm and bolting a 8" radiator fan to it, carbon fibre plenum underneath with a couple of directional vent outlets all under dash. Underdash units won't fit, so this is the current plan.

I'm also going with a generic condenser, chosen from the catalogue to fit in the crash structure along the diagonal line on the left in this pic:11151 with as big a fan as i can fit as the intercooler radiator and radiator sit on the top of this structure. I'm hoping the big fan turns off at least one of my radiator fans saving me 10 amps.

As you can see, i'm up against it. So i'd like some input as to how best to optimise the system so that i can run the aircon as much as possible without running the battery down. Currently my list of questions is this:

1). Given the data sheet for the compressor has max power at 12V, and the car runs at 14.4V, will this drop the amps at max power, or just produce more coolth? Apologies for the use of the word coolth.
2). The compressor is R134a compatible, can i get away with another refrigerant, like the hydrocarbon ones, will this increase efficiency/reduce amps/create more coolth?
3). Given the following chart:11152 obviously the better the evaporator and condenser and associated fans are, then the better off i am. So is there a better type of condenser or evaporator? Is serpentine better than plate and fin or whatever? What should i be looking for? Similarly with fans, is one brand better than the rest? My radiator fans are currently SPAL.
4). Given the above graph, can we go one better. Can we take the evaporator waste gas, which might be ten degrees, can we whack that into a heat exchanger with the condensed high pressure gas at roughly 50 degrees, just prior to the expansion valve? One like this: 11153 This will recycle the coolth left over from the evaporator and run it back into the evaporator. This appears to be the best way to have the condenser and evaporator at the same temperature, thus pushing me up that chart of COP. Problem is, will this end up cooking my compressor, max discharge temp is 130 degrees, but does it rely on some cooling from the evaporator waste? Will it matter if i'm using a better refrigerant? Can we play one off against the other?
5). Any other tips from the gurus, i'm most grateful?

[UKengineer]

Open the driver side window and drive very fast. Probably your best option!!!

[vtmpnate]

I've never seen the insides of a Lotus Elise and probably have no idea of what I'm talking about but... here goes... I would also give a serious thought on insulating those pipes going through the car. Unless using the car itself as a heat exchanger is by design on the part of the manufacturer. I live in Manila and there is probably a very good reason why no one ever thought of even using that car here.

[juk]

The Malaysian version of the car got air-conditioning, but no heater, which wouldn't have complied with the Australian Design Rules. As to insulating the pipes, i'd have to cut the sill panels off to insulate the pipes. I was thinking about carpeting the interior for insulation purposes as an alternative. All this doesn't fix the problem that it still gets up to 46 here in the shade.

[vtmpnate]

Insulating the pipes is not an option then. Some form of interior insulation may still help though. Going to your questions about condensers, evaporators, and fan assemblies... generally speaking, OEM automotive air-conditioners favor laminated evaporators and parallel-flow condensers over their serpentine counterparts for the higher efficiency (translates to faster cooling), lighter weight, and lower volume (less bulky). SPAL fan assemblies have very long service lives and are an excellent choice.

[Gary]

Hmmm... move the radiator to the back?

[vehikool]

A Sill...y idea Try cutting louvered vents in the very rear of the sills (where the pipes run) and fit a tiny fan at the front of sill to push the hot air down sill and out of rear louvered vent, make sure vent is to the rear of seat and window the warm air will be exhausted through vent, just like me. Or fit the standard Lotus Elise evap and condenser, fit a belt driven compressor at rear of seats (cut hole in floor and plate) mount compressor with pulleys as req, make up your own pipe work to suit (use aeroquip hose the best) job done should take an hour max

[juk]

Insulating the pipes is not an option then. Some form of interior insulation may still help though. Going to your questions about condensers, evaporators, and fan assemblies... generally speaking, OEM automotive air-conditioners favor laminated evaporators and parallel-flow condensers over their serpentine counterparts for the higher efficiency (translates to faster cooling), lighter weight, and lower volume (less bulky). SPAL fan assemblies have very long service lives and are an excellent choice.

Thanks for those tips, i'll keep them in mind.


Hmmm... move the radiator to the back? That's not an option, the car produces 320hp at the wheels, which provides too much waste heat to get rid of to move it to the back. I had the supercharger intercooler radiator in the back previously, and it was insufficient to remove the heat.


A Sill...y idea Try cutting louvered vents in the very rear of the sills (where the pipes run) and fit a tiny fan at the front of sill to push the hot air down sill and out of rear louvered vent, make sure vent is to the rear of seat and window the warm air will be exhausted through vent, just like me. Or fit the standard Lotus Elise evap and condenser, fit a belt driven compressor at rear of seats (cut hole in floor and plate) mount compressor with pulleys as req, make up your own pipe work to suit (use aeroquip hose the best) job done should take an hour max
There's a supercharger in the way, belt driven pump won't fit. Can't use the standard evap and condenser, new dash, heaterectomy, cost, cost, cost. It's just not an option.


Thanks for the tips and idea lads, keep them coming.

[Gary]

That's not an option, the car produces 320hp at the wheels, which provides too much waste heat to get rid of to move it to the back. I had the supercharger intercooler radiator in the back previously, and it was insufficient to remove the heat.


Sorry, but that doesn't make sense. If it can get rid of the heat when mounted in front, then why can't it get rid of the heat when mounted in back? Were you blowing hot air at it in back?

[juk]

I went from a small cheap chinese radiator in the back about 250*250mm to this:
11226
and the airflow increased at least tenfold and the surface area at least 4 fold by moving to the front because i couldn't afford to do this:
11227

[xxargs]

12V compressor you linked i data sheet is way to small in capacity, 24V compressor come more close what you need in capacity, or if you using two 12V in parallel... - ordinary car AC-compressorn have 4-6 kW cooling effect (8 degree C in evaporator) on max setting in hot environment and take 2 - 4 kW mechanical power from engine - belt drive on AC-compressor is not for fun and mostly turn more than 180 degree around pulley and hard tension to make sure enough good friction and not slip in working especially in hot environment and compressor working heavy loaded with high pressure. Compressor for car-AC is really power hungry in hot environment and take double or triple in mechanical power compare your 95 ampere generator on full load...

Ordinary Car AC-compessor itself can have pumping capacity over 10 kW in massflow on higher RPM but rest of system restrict capacity to 4-6 kW

if you still thinking electrical driven AC - start with replace your generator to bigger one - at least 200-300 ampere or so. you can also look how toyota prius electrical AC work and borrow some idea from this car against capacity needing and so on (I think toyota prius AC-compressor using lot higher Voltage than 12 Volt depend of high power consume on running and can using exist high Voltage battery bank in car)

[juk]

Tacka,

So we've got 4-6kW cooling required for a standard car, my cabin is less than half the size, but lets say i need 2-3kW. Other cars use 2-4kW to get 4-6kW, so the COP is 1.5-2. Why is that so low? How can I overcome that? If it's heat soak from under bonnet temps, i won't get that as my pipes run externally to the heat load.

According to my compressor specs on 134a i get 1.8kW cooling, however i was planning on going with hydrocarbon refrigerant and:
"A study by the School of Chemical Engineering and Industrial Chemistry, University of
New South Wales, indicates that hydrocarbon automobile air-conditioners are almost 35% more
efficient than HFC air conditioners."

and:
"the cooling capacity was 10% higher for Propane/Butane and 20% higher for cyclopropane than R12"

So 110% of 1.8kW is almost at the lower end of the 2-3kW i'm chasing.

Additionally, the hydrocarbon refrigerants respond well to superheating of gas and supercooling of liquid, so that's an option as well, but i'll leave that for a later experiment if necessary. I'll also keep 24V for later if required.