Re: [S10-EV] Fan Buffer
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Niel Nielsen
Jan 28, 2011, 1:16:41 PM1/28/11
to s10...@evchargernews.com
Being in Chicago, I see this all the time. I believe that the
function being described is not for dehumidification, but for
defrosting the front condenser coil. All home heat pumps (an air
conditioner in reverse--making the outdoors colder and the indoors
warmer) have to deal with frost and ice build-up on the outdoor
coils. When using a heat pump at lower outdoor temperatures
(typically below 45F outdoors), the temperature of the outdoor coil
will often be below 32F, and frost/ice will build up in the coils,
clogging the outdoor unit. When it senses a frost and ice buildup,
it momentarily changes the reversing valve to A/C mode (making it
hot outside and cold inside), and runs the compressor WITHOUT THE
OUTDOOR FAN RUNNING to heat up the outdoor coil to melt off the
frost/ice. It runs the indoor auxiliary heat during this time to
prevent cold air from coming out of the furnace. Overall, it's a
transfer of heat from the auxiliary heat source to the outdoor coils
to melt off the frost/ice. I have a heat pump at my work, and have
observed this often.
The truck, which also has a heat pump to heat the cab in the winter, has a similar function. But I have to say that the truck's software is not very smart about when to activate a defrost cycle. In the truck, the "outdoor" coil is the front condenser coil. To do a defrost cycle, it sets the valving to take freon (therefore heat) from the battery box and put it in the front condenser coil, and operates the compressor for 2-3 minutes, WITHOUT the condenser fan running. The battery box is pretty much the only place it can get heat from, since it wouldn't be acceptable to take it from the cab (making the cab colder in the winter time wouldn't be tolerated). The batteries are supposedly warm and have a lot of thermal mass so they can "steal" some heat to do the defrost cycle. Again, the defrost is done without the front fan operating, just like home heat pumps. If they operated the front fan during defrost, the coils would not get anywhere near as hot, and it wouldn't melt the ice.
I've never thought about that the function could be for dehumidification, but if that was the desire, they would operate the front fan to get the maximum heat transfer from the cooling coil. I'm 99% sure that the function is for defrost.
On the truck's decision on when to do a defrost cycle, it seems to be triggered by sensing the key being turned off, whenever the temperature outside is low. I don't know the exact temperature that triggers it, but it's somewhere about 40-45 F. You'd think that they would do it only if you have used the heat pump during the trip, but that's not the case--it does it whenever you turn the key off and it's cold outside. It also does a defrost even if you have used only the kerosene heater. The only thing I can figure out what the designers had in mind was the possibility that you were driving in snow/ice conditions and the condenser coil in the front got coated with snow and ice (when the condenser wasn't being used), and they want to maintain it clear of ice in case you want to use the heat pump the next time you get in the truck. They figured it wouldn't be acceptable to have to wait for a 2-3 minute defrost cycle when you want heat.
The heat pump uses about 1.5KW of power, so running it for 2-3 minutes is costing about 50-75 watt hours from the battery, which isn't much, considering a full pack in good condition is about 25,000 watt hours. But it's irritating to waste ANY battery power, and to make the batteries ANY colder than they already are, if it isn't necessary, especially in the winter when you're getting less energy/power from the batteries. Another down side of the current design is that the sensor for ambient air temperature is located right in front of the condenser coil. The decision on which heater to use depends on the temperature of this sensor, below 40F it uses the kerosene heater. After a defrost, and the vehicle is parked, the condenser coils from the defrost heat up the ambient air sensor, making it think that the ambient air is 60F rather than the 30F that it really is. Then when you get back in the truck and turn on the heat, it selects heat pump instead of the kerosene heater for the first 10 minutes, until the sensor cools back down to the true ambient air temperature. During this time, the output of the heater is essentially nothing, since the output of a heat pump is minuscule when it's 30F outside. So if you are running short trip errands on a cold day, you never get any heat in the cab because of this issue.
Gosh, it they had only made the vehicle for a little longer and had the opportunity to work out some of these simple bugs. Sigh... But bugs and all, still way better than a dinosaur juice guzzler.
Niel
The truck, which also has a heat pump to heat the cab in the winter, has a similar function. But I have to say that the truck's software is not very smart about when to activate a defrost cycle. In the truck, the "outdoor" coil is the front condenser coil. To do a defrost cycle, it sets the valving to take freon (therefore heat) from the battery box and put it in the front condenser coil, and operates the compressor for 2-3 minutes, WITHOUT the condenser fan running. The battery box is pretty much the only place it can get heat from, since it wouldn't be acceptable to take it from the cab (making the cab colder in the winter time wouldn't be tolerated). The batteries are supposedly warm and have a lot of thermal mass so they can "steal" some heat to do the defrost cycle. Again, the defrost is done without the front fan operating, just like home heat pumps. If they operated the front fan during defrost, the coils would not get anywhere near as hot, and it wouldn't melt the ice.
I've never thought about that the function could be for dehumidification, but if that was the desire, they would operate the front fan to get the maximum heat transfer from the cooling coil. I'm 99% sure that the function is for defrost.
On the truck's decision on when to do a defrost cycle, it seems to be triggered by sensing the key being turned off, whenever the temperature outside is low. I don't know the exact temperature that triggers it, but it's somewhere about 40-45 F. You'd think that they would do it only if you have used the heat pump during the trip, but that's not the case--it does it whenever you turn the key off and it's cold outside. It also does a defrost even if you have used only the kerosene heater. The only thing I can figure out what the designers had in mind was the possibility that you were driving in snow/ice conditions and the condenser coil in the front got coated with snow and ice (when the condenser wasn't being used), and they want to maintain it clear of ice in case you want to use the heat pump the next time you get in the truck. They figured it wouldn't be acceptable to have to wait for a 2-3 minute defrost cycle when you want heat.
The heat pump uses about 1.5KW of power, so running it for 2-3 minutes is costing about 50-75 watt hours from the battery, which isn't much, considering a full pack in good condition is about 25,000 watt hours. But it's irritating to waste ANY battery power, and to make the batteries ANY colder than they already are, if it isn't necessary, especially in the winter when you're getting less energy/power from the batteries. Another down side of the current design is that the sensor for ambient air temperature is located right in front of the condenser coil. The decision on which heater to use depends on the temperature of this sensor, below 40F it uses the kerosene heater. After a defrost, and the vehicle is parked, the condenser coils from the defrost heat up the ambient air sensor, making it think that the ambient air is 60F rather than the 30F that it really is. Then when you get back in the truck and turn on the heat, it selects heat pump instead of the kerosene heater for the first 10 minutes, until the sensor cools back down to the true ambient air temperature. During this time, the output of the heater is essentially nothing, since the output of a heat pump is minuscule when it's 30F outside. So if you are running short trip errands on a cold day, you never get any heat in the cab because of this issue.
Gosh, it they had only made the vehicle for a little longer and had the opportunity to work out some of these simple bugs. Sigh... But bugs and all, still way better than a dinosaur juice guzzler.
Niel
Hello Ryan
I am use to seeing this every winter.
I believe what your describing is the heat pump being used
to dehumidify the battery tray. Was the temperature about 45
degrees or less? You will not see this happen in warm
climates. The fan does not run while its performing this
function. It will run this cycle for 5 or 10 minutes after you
turn off the key. It also will dehumidify while charging under
the same conditions.
Your fan buffer might be fine and not having a blown fuse
is a sign you don't have a problem. If it warms up a bit you
can try using the AC and see if you fan comes on. Right now it
is so cold keeping the pack cool is not a problem.
Don
In a message dated 1/26/2011 8:59:29 P.M. Pacific
Standard Time, ryan...@gmail.com writes:
All,
On a very wet and humid day recently I discovered steam coming out of the front of my grill from the heat pump coil and discovered that the cooling fan wasn't spinning...
I have an OEM fan buffer (I think, at least it looks OEM) and the original fan. The buffer doesn't looked burned or split open, the fuse is still in good shape. I searched the archives and read about a workaround to activate the fan when the reversing valve is active and i am curious if that has worked for others in the past or if I need to look into re-engineering or my fan buffer.
I also read that Jeff had been replacing fan buffers on rebuilt vehicles and had a design to install a new and improved buffer system.
Any thoughts on the subject?
Ryan Dela
Beaverton, Oregon
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