Car dehumidifier

[cts_casemod]

Hi guys,

I run a 250W compressor based dehumidifier on my EV to clear up the windscreen condensation on cold/rainy days. But this is a major overkill, considering the moisture content on a small space such as a car and the fact it uses quite a bit of space on the back seat.

Using the heater works too, at the expense of far more energy, which for an EV is not always reasonable.

I've read some reviews of peltier based dehumidifiers, which are cr*p.. Not surprisingly, the commercial versions run at 60W input power! With that I was thinking about building a 48V 250W unit. Sure peltiers are inefficient, but I would need those 250W of heat power anyway, so lets consider that a bonus.

Has someone used one of the smaller versions or built a DIY? If so, exactly how much water was extracted? Or perhaps other types of dehumidifier (Desiccant for example).

Thanks

[Piwoslaw]

Here is something that will help remove moisture, without robbing your EV of precious energy:
The dash sock (silica gel to prevent frost forming on windows)

[Ryland]

Mine is 21 watts, worked great for my EV.

[redpoint5]

I just did the dash sock thing using silica gel litter. It seems to work reasonably well so far. In the 2 weeks I've had it, only twice have I noticed light fog on the windshield.

After the proof of concept is validated, I'll have my wife sew a long tube of fabric that runs the width of the dash, and a Velcro closure.

From what I've read, the gel will need to be recharged in the oven every few months. A few months gets me through winter, so this might be a once a year process for me.

The reason I resorted to this is that I'm on the very bleeding edge of being able to complete my 14 mile roundtrip commute in EV mode in the Prius. I've had 3 successful trips, but the last 2 the engine kicked on at the last half mile. I suspect loosing daylight and having to run my lights has something to do with the recent failures.

[kafer65]

This is a fantastic idea! I depend heavily on recirc air to encapsulate myself from pollen, especially in the fall to avoid rageweed pollen. Its a hateful waste to have to run AC when everything is cold at the beginning of a commute.

[cts_casemod]

Originally Posted by Piwoslaw

Here is something that will help remove moisture, without robbing your EV of precious energy:
The dash sock (silica gel to prevent frost forming on windows)

I have a couple of those with kitty litter
They do a good job removing excess moisture, but once one gets into the car the condensation from 3 people breathing quickly fogs the windscreen.

Originally Posted by redpoint5

From what I've read, the gel will need to be recharged in the oven every few months. A few months gets me through winter, so this might be a once a year process for me.

I have two large socks and recharge them every month or so, when they start to feel visibly wet. If it rains a lot (I'm in Britain) I may have to do it every fortnight. Take out the crystals and put them on a tray on the oven with the fan on for a couple hours.

I like the "All dash" idea. How many Kg does it take? I'm currently using about 1Kg on each sock. As mentioned this is just about enough to prevent condensation if it is not raining outside and I'm the only one on the car. All other cases, dehumidifier on.

Originally Posted by redpoint5

The reason I resorted to this is that I'm on the very bleeding edge of being able to complete my 14 mile roundtrip commute in EV mode in the Prius. I've had 3 successful trips, but the last 2 the engine kicked on at the last half mile. I suspect loosing daylight and having to run my lights has something to do with the recent failures.

Cool. Can you disable the DC-DC converter on the Prius at all? I have mine in manual mode for when its needed. The 40Ah LiFePO4 feeds all the 12V loads. Driving 27-30 miles seems to use about 35% of capacity at night if I keep the Blower fan off (I need to replace the variable speed resistor with a PWM circuit). The terminal voltage is also higher (stable 12.5-13V).

Originally Posted by Ryland

Mine is 21 watts, worked great for my EV.

I'm curious. Can you give some some more details? How much moisture do you remove on a typical 30m drive? or do you keep it on when plugged in?

[cts_casemod]

Originally Posted by kafer65

This is a fantastic idea! I depend heavily on recirc air to encapsulate myself from pollen, especially in the fall to avoid rageweed pollen. Its a hateful waste to have to run AC when everything is cold at the beginning of a commute.

And a waste too. The evaporator on the front is releasing all the warm air from the cabin inside

I remember when I had the air con om my older diesel turning the air con for a few seconds and magically all the condensation was gone. My electric polo has Aircon, although unused. Sadly I'm having trouble finding a DC variable speed compressor I could power from the main pack. I feel tempted to just use the dehumidifier motor with a few grams of R290. That would also give me 500W of heating, assuming a COP of 2.

Anyone successfully used an electric Prius compressor? I hear they are induction with built in inverter.

[cts_casemod]

Just wanted to update regarding this:

I tried the desiccant method with a variety of different variety of cat litters as suggested by many sources only to discover efficiency was very low. 100 or 200g of removed moisture per kg of material seemed typical.

Some of the litter cooked after a few uses and rendered unusable.

With this I got some dehumidifier bags on eBay. Something like £10 for two of them, carrying 1kg of silica.

The hardest bit was to actually regenerate the silica.
The bags hold anything from 300 to 450ml of moisture when saturated at an ambient RH > 80%. A typical electric oven is often not ventilated enough to extract this amount of water and the air inside saturates. Result is that moisture removal is very, very slow.

Running on a hot plate or a ventilated oven at 120 Centigrade seems to have done the trick, but it takes a significant amount of time and energy for each regeneration.

So anyway, what I did was to get a normal microwave and stopping it from running on steroids. I chose a magnetron power of 250W (input power = 350W). This is continuous power, not the on/off cycle to average 20% duty cycle.

The bag is warmed up for 4 minutes on a normal 900W microwave (1300W Input) and once at temperature is placed on the low power microwave for 150 minutes. I normally take it out each 45-60 minutes and shake it a bit to mix the silica. By the end the bag has anything from 0 to 50ml of water left and the maximum temperature I measured was about 110C, so no risk of damaging the silica grains (I've been testing this for about 5 regenerations so far and the collected moisture hasn't changed).

The microwave is ventilated by default, and since the electromagnetic waves heat up the water, the bag heats from the inside, where most of the humidity is.

Each of my bags collects anything from 30 to 40ml of water per day, depending on the RH. This is not enough to remove the steaming on a wet day when using the car (and bringing wet clothes/shoes), but its good enough to keep the car dry if it is not being used.

The saying that such bags can last months is a myth. After 10 days the water extraction starts to gradually fall. By the 20th day the bag is saturated, unless once pours water over it, the silica grains are not going to absorb any more moisture from the air. I normally regenerate between 1 and 2 weeks. I'm in England where RH is typically above 70% and often at 90%.