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Old 05-13-20, 09:33 PM   #1
VanSmith
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Default Electric Van Heat Pump/AC (and more!)

Hello All,

I am new to the forum, so forgive me if I have missed an intro, but I searched around and did not find info on this exact product. Please help guide me as you see fit. My background is as a mechanical engineer who dabbles in electronics and control systems.

I am working on a new van build project where the idea is to get as close to an all-electric van as possible in perpetration for an all electric future of travel. The main specifications are a 1.2 kW solar array (possibly more, with a solar awning) powering a 24.8 kwh diy tesla pack (4 p100d model s modules).

Of main concern is heating and cooling. As a base, I would like to build an AC/heatpump that will work off of 48v DC power. Has anyone here done this before? I see some home based systems with out of this world SEER and COP, and would like to get as close as possible.

As a secondary goal, I would like to emulate Tesla, and have Refrigerant -> Glycol HX on both high and low sides, to move energy around in an intelligent manor in order to simultaneously 1) cool a fridge 2) make domestic hot water.

My background in refrigeration is limited, but I can solder and build.

My last question is seemingly very basic. What does it take to turn an AC unit into a heat pump? I is as simple as running the cycle in reverse (swapping power leads..)?


Thanks in advance to whomever got though my wall of text.

Mike

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Old 05-15-20, 11:54 AM   #2
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Interesting project Mike!

What about using a high voltage heat pump from a Nissan Leaf?
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Old 05-16-20, 10:51 AM   #3
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There are a few obstacles and hurdles to overcome with automotive systems. Cooling mode has been practicing for 50 plus years in cars, so it is the easy part. The challenges come with heating mode.

I encourage you to read the thread in this group entitled "heat pumps for dummies". Not saying anything about you, but we threw it together to help people quickly get a grasp on the concepts and terms involved. Especially from a DIY approach.

Traditional automotive systems have been driven mechanically by a belt and pulley from the rotating engine. The new EV designs are using an electronic control and an electric compressor. I'm not familiar with the new compressor designs, so any background information here would be appreciated. I do know that the BMW, Tesla, Nissan, Toyota and a few other designs are using heat pump technology for heating.

In the proven and practical designs, the reversal of refrigerant flow from heating to cooling mode is accomplished not by reversing the compressor rotation, but by electric solenoid valves. With most split and minispli home systems, there is a single 4 way reversing valve that reverses the direction of refrigerant flow on command. With the automotive designs I've seen, they use more than one valve to accomplish this task. The main reason for the multiple valve approach is because they employ different strategies to heat, depending on environmental conditions and climate control settings.
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Old 05-16-20, 04:18 PM   #4
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so here's a generic diagram of a heat pump:


The cooling coil is the evaporator. It's attached to the suction side of the compressor, so the working pressure is low. High pressure liquid refrigerant is forced through a restriction, and the resulting pressure drop forces the refrigerant to boil off (evaporate) at constant temperature until it becomes gaseous. The phase change requires energy, so it is absorbed from the surrounding environment. Not a big deal to refrigerate glycol mixture, just make sure it can't freeze.

The condenser heat exchanger is fed by the hot high pressure output of the compressor. The hot gas is cooled down below it's boiling point and changes to the liquid phase. The heat gained from boiling is released at constant temperature until the gas has all condensed into the liquid phase.

Both of these phase change processes happen at a constant temperature, depending on the refrigerant pressure. This is known as the "latent" portion of the heat flow, and maximizing it will increase your efficiency. Once all the refrigerant has completely vaporized or condensed, a little heat flow forces a temperature change towards ambient temperature and the heat transfer is done for the most part. This is known as "sensible" heat transfer. You need a little of it to keep the gas gas and the liquid liquid (compressors don't like pumping liquid and expansion valves don't like gases), but not very much.

The thing with the EV heat pump systems is that they have more than two heat exchangers. Outside the cabin, they have one for air to flow through like most cars, plus a transmission cooler and maybe a battery heater/cooler. Inside, they have a separate evaporator and condenser, plus maybe a dehumidifier as well. As with most modern cars, everything has sensors and valves, plus a brain box controlling the system.

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Old 06-14-20, 06:35 PM   #5
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You could use something like this for the heat pump:
https://www.hotspotenergy.com/DC-air-conditioner/
they also have a solar direct model.
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Old 07-01-20, 03:51 PM   #6
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There are some newly available small DC compressors mostly from chinese sources for things like this. Much of it is 24v but I'm pretty sure they make 48v compressors too. From there I would rather make my own system from scratch around it. Many of them are R134A, so it's easy/cheap to get refrigerant. I have been thinking about something similar - I want electric A/C in my Jeep so I can keep it cool from a few KWH of lithium batteries. Would be amazing to have A/C without the engine running for at least an hour.
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Old 07-01-20, 08:57 PM   #7
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Quote:
Originally Posted by AirConditioner View Post
I have been thinking about something similar - I want electric A/C in my Jeep so I can keep it cool from a few KWH of lithium batteries. Would be amazing to have A/C without the engine running for at least an hour.
Don't bother, ice storage ends up with a very similar amount of cooling per pound of energy storage medium. Unless you're converting it to a plug in hybrid, that is...
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Old 07-03-20, 07:56 AM   #8
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Quote:
Originally Posted by NiHaoMike View Post
Don't bother, ice storage ends up with a very similar amount of cooling per pound of energy storage medium. Unless you're converting it to a plug in hybrid, that is...
Not sure what you drive but I'm wondering where you keep a ton of ice stored? Thermal storage is amazing for housing, not so much being mobile.

I'm building a ~4kwh lithium battery that goes inside under the back seat of my Jeep. It's going to replace my normal lead starting battery under the hood, as well as giving me extra capacity to run an inverter.

If I already have the battery capacity, I could setup maybe 1 ton of A/C and easily run that from the battery for an hour or more. Either pre-cool before I get in or let it run in a parking lot somewhere. Great if pets/people/groceries need to remain in the vehicle for a little while. Being a heat pump would be even better. The hard part is where to mount it...
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Old 07-04-20, 02:10 PM   #9
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1kg of ice will absorb 333kJ of energy. 1kg of the best lithium batteries will store on the order of 800kJ of energy, but cheaper ones will store considerably less. Then there's the efficiency of the heat pump which is already going to be lower than average given the space and weight limitations - COP of 2 would be optimistic.

Doesn't seem worth the cost to get a mere 4-5x the storage per weight even when the best parts are used. Also, making heat pumps smaller makes it more difficult to make them efficient while the efficiency of "discharging" thermal storage remains high until insulation losses become significant.

EDIT: Also, batteries have to be derated to get good lifetime out of them - on the order of 20-80% for lithium leaving 60% of capacity usable. A thermal storage tank can be left at any "state of charge" for an indefinite time with no impact on lifespan.
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Old 07-06-20, 04:25 AM   #10
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I'm not sure why you are arguing. What you are talking about is in no way helpful or relevant to air conditioning a "van" or a Jeep. There is nowhere to store ice, and even if there was you would have to refill it. An air conditioning system provides cold air on demand at any time, and is unlimited for as long as you have power or gas.

Weight vs weight doesn't mean anything. As I said, I'm already installing a large battery for other purposes, and any sort of recreational vehicle needs to have more than just a starting battery also. If the power is there (literally 120v outlets) then an A/C compressor is a wonderful thing. But if you are concerned about weight, the lithium battery will save me about 30lbs over the original lead acid. I'm not here to discus battery technology.

It's hard to find a place to mount an electric A/C system in a vehicle, but certainly a hell of a lot easier than storing ice. I honestly can't comprehend what you are arguing about. Nobody uses ice to cool a car. That is not even remotely practical.

If you're talking about heating/cooling a house, then yes, definitely water and ice have way better storage capacities than batteries for less money. This is totally different than what you need in a vehicle.

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