EcoRenovator - View Single Post

DEnd · 06-29-16, 02:50 AM

Ok so I am far from an expert in HVAC and dehumidification so take what I say with a grain of salt (pun intended).

Your initial idea of running a GSHP with the goal of reducing the latent heat load on your A/C unit is basically a good one. You get a rise in Coefficient Of Performance (COP) by lowering the temperature of where you are pumping heat to. This is the same effect but in reverse as you saw with switching the heat source (where you are pumping heat from) of your heat pump water heater. You were also planning on increasing system efficiency by keeping the heat out of the house (by placing the compressor and condenser outside). This is also a very good idea as a stand alone dehumidifier is basically a 110% efficient (or higher) electric heater.

Now let us take a look at what happens with your system design if it is working in such a way that it removes all the water it possibly can from the air that enters it for a given evaporator temperature, let's say that temperature is 33°F. So what happens? The air enters the system at say 75°F and 60% Relative Humidity (RH). When it leaves the system it will be 33°F and 100% RH. This is because RH is relative, the percentage is the percentage of the amount of water the air can hold at that temperature.

This is not a very efficient way to operate a dehumidifier. Why? Because we are removing sensible heat also, not just latent heat. We we want to use a dehumidifier to dehumidify the air, not to cool it. We want to remove latent heat only... So how can we reduce the amount of sensible heat that is lost to the dehumidifier? Well if we remember that heat flows from high to low we see there is a temperature difference between the air flows. Ideally we want the temp of the air coming out of the dehumidifier at 75°F, not 33°F. Since the air entering is at 75°F already we can use that energy to warm up the air leaving right? There are multiple ways to do this, air-air heat exchanger, heat pump, heat pipes, etc...

An interesting thing happens when we do that. The Relative Humidity of the air going to the evaporator rises with the temperature drop. Ideally the temperature would drop to 33°F raising the RH to 100% before it gets to the evaporator now the heat pump in the dehumidifier is only moving Latent heat. Now this is impossible to achieve, but the closer we can get the air going into the evaporator to 100% humidity the more efficient it becomes at removing water from the air.

Now let us look at this from another angle. The refrigerant in the system has to be compressed in order to raise its temperature, it needs to be compressed enough that its temperature becomes higher than where the condenser is. The more we have to compress it to reach that temperature the more work the compressor has to do, thus using more energy. What we really want is the temperature difference between the evaporator and the condenser to be as low as possible. We can't affect the temperature of where we are pumping heat to and thus we can't control the ideal condenser temperature. We can control the temperature of the evaporator however, this is because we know that a dehumidifier is more efficient the closer to 100% RH the air entering the evaporator is. If we can raise the RH to 100% (no matter what the temperature is) then the evaporator can be at any temp.

So with our system instead of moving heat from the air going into the evaporator to the air coming out of the evaporator, what we really want to do is move moisture from the air coming out of the evaporator to air coming into the evaporator. Fortunately there are ways to do this, an energy recovery air to air exchanger is one way this is basically a material that can absorb water yet block air. Alternatively we can move a desiccant (a material that uses sorption to collect water from a substance, in this case air) between the air flows. When a desiccant becomes saturated it will give off a portion of its water to an air flow if the RH of that air flow is less than 100%. The common ways to do this are with a desiccant water fall or a desiccant wheel. Realistically the air going into the evaporator from the desiccant can never be at 100% RH, just like we can never move all the heat from in incoming air stream to the out going air stream.

Why not just use a desiccant and forget using any refrigeration equipment in the dehumidifying process? I mean we can move it outside and heat it up right? The issues with that are #1 controlling the dehumidification, and #2 we will need to dehumidify when there isn't solar energy to dry the desiccant.

Theoretically though you could use a large storage tank to hold a liquid desiccant and then do an indoor waterfall and an outdoor one in a solar oven. Given enough capacity of the tank you should be able to have a big enough buffer to cover the no solar times. The other issue is you would end up pumping heat into the house that way, likely more heat than you'd have with just using a dehumidifier.

06-29-16, 02:50 AM	#18
DEnd Apprentice EcoRenovator Join Date: Nov 2011 Location: NC Posts: 117 Thanks: 6 Thanked 27 Times in 25 Posts	Ok so I am far from an expert in HVAC and dehumidification so take what I say with a grain of salt (pun intended). Your initial idea of running a GSHP with the goal of reducing the latent heat load on your A/C unit is basically a good one. You get a rise in Coefficient Of Performance (COP) by lowering the temperature of where you are pumping heat to. This is the same effect but in reverse as you saw with switching the heat source (where you are pumping heat from) of your heat pump water heater. You were also planning on increasing system efficiency by keeping the heat out of the house (by placing the compressor and condenser outside). This is also a very good idea as a stand alone dehumidifier is basically a 110% efficient (or higher) electric heater. Now let us take a look at what happens with your system design if it is working in such a way that it removes all the water it possibly can from the air that enters it for a given evaporator temperature, let's say that temperature is 33°F. So what happens? The air enters the system at say 75°F and 60% Relative Humidity (RH). When it leaves the system it will be 33°F and 100% RH. This is because RH is relative, the percentage is the percentage of the amount of water the air can hold at that temperature. This is not a very efficient way to operate a dehumidifier. Why? Because we are removing sensible heat also, not just latent heat. We we want to use a dehumidifier to dehumidify the air, not to cool it. We want to remove latent heat only... So how can we reduce the amount of sensible heat that is lost to the dehumidifier? Well if we remember that heat flows from high to low we see there is a temperature difference between the air flows. Ideally we want the temp of the air coming out of the dehumidifier at 75°F, not 33°F. Since the air entering is at 75°F already we can use that energy to warm up the air leaving right? There are multiple ways to do this, air-air heat exchanger, heat pump, heat pipes, etc... An interesting thing happens when we do that. The Relative Humidity of the air going to the evaporator rises with the temperature drop. Ideally the temperature would drop to 33°F raising the RH to 100% before it gets to the evaporator now the heat pump in the dehumidifier is only moving Latent heat. Now this is impossible to achieve, but the closer we can get the air going into the evaporator to 100% humidity the more efficient it becomes at removing water from the air. Now let us look at this from another angle. The refrigerant in the system has to be compressed in order to raise its temperature, it needs to be compressed enough that its temperature becomes higher than where the condenser is. The more we have to compress it to reach that temperature the more work the compressor has to do, thus using more energy. What we really want is the temperature difference between the evaporator and the condenser to be as low as possible. We can't affect the temperature of where we are pumping heat to and thus we can't control the ideal condenser temperature. We can control the temperature of the evaporator however, this is because we know that a dehumidifier is more efficient the closer to 100% RH the air entering the evaporator is. If we can raise the RH to 100% (no matter what the temperature is) then the evaporator can be at any temp. So with our system instead of moving heat from the air going into the evaporator to the air coming out of the evaporator, what we really want to do is move moisture from the air coming out of the evaporator to air coming into the evaporator. Fortunately there are ways to do this, an energy recovery air to air exchanger is one way this is basically a material that can absorb water yet block air. Alternatively we can move a desiccant (a material that uses sorption to collect water from a substance, in this case air) between the air flows. When a desiccant becomes saturated it will give off a portion of its water to an air flow if the RH of that air flow is less than 100%. The common ways to do this are with a desiccant water fall or a desiccant wheel. Realistically the air going into the evaporator from the desiccant can never be at 100% RH, just like we can never move all the heat from in incoming air stream to the out going air stream. Why not just use a desiccant and forget using any refrigeration equipment in the dehumidifying process? I mean we can move it outside and heat it up right? The issues with that are #1 controlling the dehumidification, and #2 we will need to dehumidify when there isn't solar energy to dry the desiccant. Theoretically though you could use a large storage tank to hold a liquid desiccant and then do an indoor waterfall and an outdoor one in a solar oven. Given enough capacity of the tank you should be able to have a big enough buffer to cover the no solar times. The other issue is you would end up pumping heat into the house that way, likely more heat than you'd have with just using a dehumidifier.