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VanSmith 05-13-20 08:33 PM

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

Daox 05-15-20 10:54 AM

Interesting project Mike!

What about using a high voltage heat pump from a Nissan Leaf?

jeff5may 05-16-20 09:51 AM

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.

jeff5may 05-16-20 03:18 PM

so here's a generic diagram of a heat pump:
https://www.everysolarthing.com/stat...-heat-pump.jpg

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.

Weather Spotter 06-14-20 05:35 PM

You could use something like this for the heat pump:
https://www.hotspotenergy.com/DC-air-conditioner/
they also have a solar direct model.

AirConditioner 07-01-20 02:51 PM

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.

NiHaoMike 07-01-20 07:57 PM

Quote:

Originally Posted by AirConditioner (Post 62773)
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...

AirConditioner 07-03-20 06:56 AM

Quote:

Originally Posted by NiHaoMike (Post 62774)
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? :confused: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...

NiHaoMike 07-04-20 01:10 PM

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.

AirConditioner 07-06-20 03:25 AM

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.

NiHaoMike 07-06-20 07:54 AM

Quote:

Originally Posted by AirConditioner (Post 62786)
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.

The thermal storage can be "recharged" with an onboard heat pump. Most likely one based on a 5000BTU/hr window A/C since it's hard to go smaller with decent efficiency and still keep the cost of parts low, while going bigger increases weight and size. (And are you planning to have more than 500W or so of solar panels to be able to power a larger A/C constantly? Might be reasonable for a RV but not a van.)

4kWh of batteries would be overkill unless it's also being used for traction power. Could probably go that route fairly easily by replacing the alternator with a BLDC motor/generator.

AirConditioner 07-06-20 08:28 PM

Quote:

Originally Posted by NiHaoMike (Post 62787)
The thermal storage can be "recharged" with an onboard heat pump.

Thermal storage doesn't work on wheels. I am seriously baffled by your posts...

Where would you suggest one mount this thermal storage tank in a Van or Jeep? It is not even remotely practical, and moving all that mass just uses more energy.

Quote:

Originally Posted by NiHaoMike (Post 62787)
(And are you planning to have more than 500W or so of solar panels to be able to power a larger A/C constantly? Might be reasonable for a RV but not a van.)

I'm not planning on any solar panels, though the OP said he was going to have 1.2KW of panels, which is more than enough to run a ton of A/C and charge batteries at the same time.

I am planning on putting a 4KWH battery under the back seat, for numerous reasons portable power is extremely convenient. The OP says he has a 24.8 KWH battery. That can run a LOT of air conditioning, or other useful things like power inverters, lights, heating, cooking, fans, etc. With only 4KWH I can run a ton of A/C for 4 hours...or smaller A/C for even longer. The lithium battery will also be used to run the block heater in the winter without an extension cord, and removing the lead starting battery gives me more room under the hood.

I'm not trying to be rude but I honestly can't understand why your posts have anything to do with the topic. I don't think you understand the question. Onboard battery power is wonderful to have in any vehicle for numerous reasons. Thermal storage to any useful degree simply isn't an option on wheels. Thermal storage is a wonderful thing in stationary applications, but it is always very large and very heavy.

NiHaoMike 07-06-20 09:17 PM

Quote:

Originally Posted by AirConditioner (Post 62788)
Where would you suggest one mount this thermal storage tank in a Van or Jeep? It is not even remotely practical, and moving all that mass just uses more energy.

It takes energy to move batteries as well, although I suppose that can be largely offset with regenerative braking. It's down to the overall cost vs benefit of a large battery pack vs a smaller battery pack plus thermal storage.
Quote:

The OP says he has a 24.8 KWH battery. That can run a LOT of air conditioning, or other useful things like power inverters, lights, heating, cooking, fans, etc.
He's building an EV or plug in hybrid, a different use case. Of course there needs to be a sizable battery for that application.

jeff5may 07-07-20 01:22 AM

Sounds like you are on the right track already. A ton of a/c is literally enough for a Winnebago. In a Jeep, I'd probably go with half a ton of capacity, and get something energy star. They run part time pretty well and shut down the compressor before the fan to get the cool air out while the system bleeds down.

If you are running a bang bang unit (non vrf), make sure the power inverter has enough balls to start the compressor. It draws locked rotor amps for a few line cycles every startup. As close to pure sine wave as you care to spend will pay back in efficiency, extending the run time. At that capacity, I don't know if the price difference would justify a vrf unit. Sure, you'd save money for a smaller power inverter, but the half ton window shakers are selling for around 75 bucks used here. Maybe 150 or so brand new from the usual box stores.

AirConditioner 07-14-20 04:25 PM

Quote:

Originally Posted by NiHaoMike (Post 62789)
He's building an EV or plug in hybrid, a different use case. Of course there needs to be a sizable battery for that application.

A "future" EV. And that means less room and cargo capacity for thermal mass. Just drop it. You cannot have thermal mass in a vehicle. Jesus...you seem like a smart guy, but you're just talking stupid here. Lithium batteries and fossil fuel make more compact cold storage than anything else...except maybe a bottle of CO2.


Quote:

Originally Posted by jeff5may (Post 62792)
At that capacity, I don't know if the price difference would justify a vrf unit. Sure, you'd save money for a smaller power inverter, but the half ton window shakers are selling for around 75 bucks used here. Maybe 150 or so brand new from the usual box stores.

That seems to be an issue I run into a lot. Those cheap little units are SO hard to beat for the parts inside! I've been able to start a 5k BTU from as small as an 800 watt very old modified sine inverter. The compressor makes an odd humming noise on the inverter but I'm sure it would work better with a 2-3kw pure sine wave.

I have heard people claim that when using compressors that were not designed to be mounted on vehicles they die early because they can't handle the vibrations as well. I've seen this as a complaint on RV forums with mini splits. With that said I have no idea what would make an "RV" compressor different, but I know they cost a lot more.

I really like the new DC powered compressors I've seen from China...but, they still seem to cost $500 or more for very very small, incomplete system.

Also if someone is doing something like a van, they're making pure DC mini splits now for direct solar applications in pretty small sizes. Gotta pay to play though.

NiHaoMike 07-14-20 10:50 PM

Quote:

Originally Posted by AirConditioner (Post 62823)
A "future" EV. And that means less room and cargo capacity for thermal mass. Just drop it. You cannot have thermal mass in a vehicle. Jesus...you seem like a smart guy, but you're just talking stupid here. Lithium batteries and fossil fuel make more compact cold storage than anything else...except maybe a bottle of CO2.

For the batteries, a mere 4-5x per pound compared to ice which actually gets reduced to about 2.5x after derating the batteries. Doesn't seem like a huge improvement compared to the cost. On the other hand, you cannot regen into ice storage at any meaningful rate or use it for anything other than cooling, so that's where there's value in batteries.

It's hard to beat the energy per weight of running a generator, but there are a number of obvious disadvantages to that which relegate it to more of a backup application. The real game changer would be fuel cells - quiet and efficient, just needs the cost to come way down.
Quote:

That seems to be an issue I run into a lot. Those cheap little units are SO hard to beat for the parts inside! I've been able to start a 5k BTU from as small as an 800 watt very old modified sine inverter. The compressor makes an odd humming noise on the inverter but I'm sure it would work better with a 2-3kw pure sine wave.

I have heard people claim that when using compressors that were not designed to be mounted on vehicles they die early because they can't handle the vibrations as well. I've seen this as a complaint on RV forums with mini splits. With that said I have no idea what would make an "RV" compressor different, but I know they cost a lot more.

I really like the new DC powered compressors I've seen from China...but, they still seem to cost $500 or more for very very small, incomplete system.

Also if someone is doing something like a van, they're making pure DC mini splits now for direct solar applications in pretty small sizes. Gotta pay to play though.
You can take a look at repurposing a Prius compressor, you'll probably want the Gen3 since that includes the drive circuits.
https://openinverter.org/forum/viewtopic.php?f=14&t=258
There is a disadvantage in that they need about 200V to work, you can either build the battery to run in that voltage range (good idea if you want to be able to regen at high rates without dealing with really high currents) or use a DC/DC converter to get that voltage. It helps that the DC bus in a 120V inverter will be about 180V which probably will work.

Elcam84 07-20-20 07:26 PM

FYI the AC system in the average car is around 3 tons. Some smaller some larger. For a van you will be on the larger end. So you would need 3+ tons of ice per hour to cool it...

So to keep the van cool for say 8 hours you would need 24 tons of ice to do it. Actually more since some of that ice will have melted before you got to use it. So if it's a bigger van you could end up needing say 30 tons of ice. It would be more efficient to go back to hauling ice from frozen rivers up north on ships and using it to cool than it would to produce the ice and then haul it in a trailer behind the van.


If you want to cool it and your power source is electricity the solution is simple. Order a Sanden electric compressor. Sanden is the top brand of automotive AC compressors on the market. Used in everything from production vehicles to big dozers and other heavy equipment.

NiHaoMike 07-20-20 08:16 PM

Quote:

Originally Posted by Elcam84 (Post 62850)
FYI the AC system in the average car is around 3 tons. Some smaller some larger. For a van you will be on the larger end. So you would need 3+ tons of ice per hour to cool it...

Tons per 24 hours, and only for the short time it takes to cool down. As mentioned, 1 ton is plenty for a fairly large RV.

Elcam84 07-20-20 10:41 PM

My goof and typed that wrong.

The typical factory AC in a car is usually rated at around 3 tons. Now if you are doing a van and it's been insulated etc then yeah you can get away with less. Production cars have no real thermal insulation so massive amounts of heat gain to them.

Rooftop RV units are usually around 15Kbtu.

jeff5may 07-30-20 01:56 PM

This debate about ice storage versus battery range is getting ridiculous! The number one challenge for current BEV solutions is range. More range=better is the rule. Anything that adds mass adds drag on the range, period. So consider this, Batman: manufacturing ice on the fly at max capacity would cost you less range than carrying a big ice cube from the starting point. With a ton or two of capacity, it doesn't take an hour to make 100 gallons of ice. NO EXTRA BAGGAGE!

I have an idea, we could go to the dollar store and buy a whole row of the 6 dollar magic air conditioner EVAP and mount them on the roof. That could subcool the refrigerant while parked or driving and DOMINATE! Or hey, why not just put PCM sound deadening interior panels everywhere, with a heat soak range of 70 to 75 degrees. I dub thee" sta-cool" interior panels.

Just kidding, don't listen to my ideas...

NiHaoMike 07-30-20 07:02 PM

Quote:

Originally Posted by jeff5may (Post 62893)
With a ton or two of capacity, it doesn't take an hour to make 100 gallons of ice. NO EXTRA BAGGAGE!

2 tons of refrigeration operating for one hour will only make 183 pounds of ice, or about 23 gallons.

What is the end goal now? Are we building an EV that regularly gets driven (in which case extra batteries definitely wins) or a RV/van that stays parked for extended times?

jeff5may 07-31-20 10:40 AM

^What Mike said^

"Only 180 pounds" is a lot of baggage. It's like an extra passenger. Per hour. Also consider that ice cubes stack up to about 1/3 water, 2/3 air. I dare you to go to a hotel and try to get 15 pounds of ice cubes into the 2 gallon bucket they give you. Who in their right mind would freeze up a solid block of ice and then wrangle it into a vehicle? Why not add 100 pounds of equipment and generate twice the weight per hour in cold? I'm done with this argument.


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