Saving Money With Pre-Cooling

[apostolakisl]

Guys,
 
There is no way that jumpering power from one house to another is going to be legal.  Even if the power co has no tos against that, it would almost certainly violate building/fire codes with two houses flipping back and forth between two electrical services.  Whenever changing the electrical around on the main service panel, you are required to get a permit, and there is no way they would pass this.
 
Now if you wanted to pump chilled water or something from one house to the other, I doubt they could stop that.

 

[jhdale]

The "bioPCM" gel in the university building, derived from vegetable oils, will be "charged" each night when windows automatically open to flush the building with cold outdoor air. The solid gel then absorbs heat as it melts the next day.

I wonder what the consequences are for fire safety. I have a cooling vest which works on the same principle, filled with a paraffin mixture that freezes at 58 F (so you can quickly freeze it by tossing in a cooler with some ice). Works pretty well. But with a paraffin base I imagine it's flammable. I'm not worried wearing it but if you filled the walls with it, I would think that could make it a lot harder to extinguish a building fire. Since the material by design has to liquefy, it could also leak out and get into other parts of the walls.

It's possible what they are using is not flammable, but "derived from vegetable oils" makes me suspicious.

 

[Automate]

They've done the fire testing http://www.phasechange.com/index.php/en/information/fire-testing
A lot of other good info on their site.  If anyone gets pricing please post it because I am curious.

 

[jhdale]

They've done the fire testing http://www.phasechange.com/index.php/en/information/fire-testing
A lot of other good info on their site.  If anyone gets pricing please post it because I am curious.

Cool, thanks for the link.

 

[apostolakisl]

They've done the fire testing http://www.phasechange.com/index.php/en/information/fire-testing
A lot of other good info on their site.  If anyone gets pricing please post it because I am curious.

 
I'm going to take a guess that it is big bucks.  Mostly based on the lack of popularity. Also could just be the fact that builders in general hate new technology since it sometimes comes back to bite them in the ass when some flaw is found 5 or 10 years down the road.
 
Looking over the info, not sure that you could use it much around here, at least not on a roof.  They have 3 products, 23c, 25c, and 27c.  For a lot of the summer, the nights don't drop below 27c, so it would never solidify and you would only get the regular specific heat thermal mass, not the phase change thermal mass.  This stuff sure seems like it would work great in a desert with the hot days and cold nights.

 

[Automate]

Looking over the info, not sure that you could use it much around here, at least not on a roof.  They have 3 products, 23c, 25c, and 27c.  For a lot of the summer, the nights don't drop below 27c, so it would never solidify and you would only get the regular specific heat thermal mass, not the phase change thermal mass.  This stuff sure seems like it would work great in a desert with the hot days and cold nights.

 
You could use it for load shifting even if the outside air does not get that cold.  Place some of the material inside of a HVAC duct or between the outside of the duct and its insulation.  Then run the AC during the off-peak night and morning to solidify the PCM.  Then when you get into the high rate period run the HVAC circulation fan only to cool the house while the solid turns into liquid.  With 23C it would not feel real cold but the fan would circulate the cool air throughout the house.
 
They mention the inside duct option for their ThermaStix product here http://www.phasechange.com/index.php/en/products/therma-stix
 
For a home you would not need that much material so it may not be that expensive for the quantity required.  The problem may be that they may not want to sell small quantities to an individual..

 

[apostolakisl]

You could use it for load shifting even if the outside air does not get that cold.  Place some of the material inside of a HVAC duct or between the outside of the duct and its insulation.  Then run the AC during the off-peak night and morning to solidify the PCM.  Then when you get into the high rate period run the HVAC circulation fan only to cool the house while the solid turns into liquid.  With 23C it would not feel real cold but the fan would circulate the cool air throughout the house.
 
They mention the inside duct option for their ThermaStix product here http://www.phasechange.com/index.php/en/products/therma-stix
 
For a home you would not need that much material so it may not be that expensive for the quantity required.  The problem may be that they may not want to sell small quantities to an individual..

 
They have a whole roofing system that they were showing, which is what I was referring to.
 
I didn't see any setups where you blow air over the product (like by lining ducts).  I'm not sure you would get much result from that.  To be of any value the house air entering the intakes would have to be pretty warm for there to be much heat extraction.  Of course this means you would have to be living in a house that was pretty hot, I'm thinking well into the 80's with the 73 degree melting point material.  Maybe if they had a product that solidified around 50 degrees.  But You would also now be dealing with condensation.
 
I agree, it would work as thermal mass in a precooling situation (load shifting/spreading).  You would put it on the inside surfaces of the exterior walls/ceilings of the house.  Drop your house to 72 at night using the 75 degree (25c) material and it should solidify.  Then during the day it will melt as the heat tries to migrate in from the outside, holding the interior surface temp of the exterior parts of the house at 75.  Of course this is not nearly as beneficial as if you could get the outside air to solidify the product.  You are still burning energy to run your AC to solidify the product.  It will take longer to precool the house since you have all the extra thermal mass.  On the plus side though, you will also be spreading out the hvac load over a greater period of time, thereby allowing a smaller unit.  Theoretically, it would be most efficient (not on a $ basis, but total kwh basis) to size an hvac system such that it runs 24/7 and when doing so, the material just barely melts all the way at its hottest and then just barely finishes re solidifying when the thing starts heating up again.  But since electric rates are higher during the day, the Phoenix folks would aim to be able to shut the unit off during those hours. 
 
It seems to me that they need a version of this product that changes state around 100.  Then you could do a roof sandwich of 100 degree stuff on the outside just under the roofing material, insulation, 75 degree stuff on the inside.  Just as a point of reference, my roof surface temp will get as high as 155 under direct 3pm sunlight when it is 105 outside.  So locking that temp in at 100 could be huge, and I would think it would still get enough hours of cooling to solidify at night.
 
I do say, this is pretty interesting stuff.

 

[Automate]

I didn't see any setups where you blow air over the product (like by lining ducts).

 
Quote "Drop in thermal mass that can be inserted into duct work to store nightime cool enegry." from the link I gave above
 
OP stated he lets his house get up above 80 deg during the peak demand period.  73 should feel cool compared to 80.  Also, the 23C is their "standard" product, they can make it with other melting points but I'm sure it would be more expensive and they probably require some minimal order.
 
Quote "We make ThermaStix custom for individual projects so they are available for order in a wide range of transition temperatures, or "Q-values".
 
I don't see the roof system being that great except for maybe when you have a situation where you don't have enough space for proper insulation.    I think more insulation or a ventilation fan is always going to be more cost effective than PCM for an attic.  With a ventilation fan I have at the top of my attic it only gets a few degrees over outside temperature down at the bottom of the attic where the insulation is.

 

[apostolakisl]

Quote "Drop in thermal mass that can be inserted into duct work to store nightime cool enegry." from the link I gave above
 
OP stated he lets his house get up above 80 deg during the peak demand period.  73 should feel cool compared to 80.  Also, the 23C is their "standard" product, they can make it with other melting points but I'm sure it would be more expensive and they probably require some minimal order.
 
Quote "We make ThermaStix custom for individual projects so they are available for order in a wide range of transition temperatures, or "Q-values".
 
I don't see the roof system being that great except for maybe when you have a situation where you don't have enough space for proper insulation.    I think more insulation or a ventilation fan is always going to be more cost effective than PCM for an attic.  With a ventilation fan I have at the top of my attic it only gets a few degrees over outside temperature down at the bottom of the attic where the insulation is.

 
OK, so I see the sticks, but I have serious doubts about that working.  Your hvac blower is sized to move enough air to keep your house 70 with 40 degree air.  They aren't sized to keep your house 80 with 75 degree air.  My suspicion is that your cooling/energy efficiency losses from running the fan would make those an overall loser.
 
Using attic fans works well, but you are using significant electricity to move enough air to keep an attic as cool as the outside air.  This would use zero energy and would work just as well where there is no attic or where the attic is conditioned space (like my house).

 

[Automate]

OK, so I see the sticks, but I have serious doubts about that working.  Your hvac blower is sized to move enough air to keep your house 70 with 40 degree air.  They aren't sized to keep your house 80 with 75 degree air.  My suspicion is that your cooling/energy efficiency losses from running the fan would make those an overall loser.

 
Yes, but keep in mind you are not trying to cool the house.  The whole house is already pre-cooled.  You are only trying to extend the time without running AC.  Not eliminate it.
ECM circulation fans use very low amounts of power.  You don't need a lot of air flow since your not trying to keep the AC coil from freezing over.
 
 

Using attic fans works well, but you are using significant electricity to move enough air to keep an attic as cool as the outside air.

 
If I run my attic vent fan on high speed for 12 hours at 10 cents per kWH it cost me 23 cents and its a PSC motor.

 

[apostolakisl]

Yes, but keep in mind you are not trying to cool the house.  The whole house is already pre-cooled.  You are only trying to extend the time without running AC.  Not eliminate it.
ECM circulation fans use very low amounts of power.  You don't need a lot of air flow since your not trying to keep the AC coil from freezing over.
 
 
 
If I run my attic vent fan on high speed for 12 hours at 10 cents per kWH it cost me 23 cents and its a PSC motor.

 
A typical hvac fan in a 3 or 4 ton system is going to use around 500 watts.  At his daytime electric rate, that is going to add up fast.  And if you have 2 of them count it as 1000 watts.  Not only the 1000 watts in electric cost, but all of that turns into heat. Now you need 100% efficient transfer of heat from the air into those sticks. .. which isn't even going to come close to happening, they are sticks, they have no heat exchange fins or anything.  I doubt the air coming out the vents would even be 1 degree colder than when it went in.  You would really be better off running a ceiling fan over your head for some wind chill.
 
Now consider that those sticks are all warmed up when you go to low rate time in the evening.  You flip on the compressor.  But instead of nice 40 degree air, you get warmer air since the sticks are dumping heat into the air.  So now you were hoping to at least get the house cooled down by bedtime, but instead the thermal mass of these sticks is blocking rapid cooling of the house.
 
You could put the sticks on the return side, but in that case you would have to get your house down to 70 or so for quite a few hours to get those sticks to solidify again since the return air (if your system is designed with optimal return air intakes) is sucking up the hottest air in the house.  They really don't "freeze" until the temp is a couple degrees below their set-point, if you look at the graphs on their website.  If it took 10 hours to melt the sticks at 80, then it should take 10 hours to freeze them at 70.
 
I'm telling you, this isn't going to work very well at all.
 
And I don't know how you are getting your attic down to outside temp with just a single 100 watt exhaust fan.  No one around here gets that kind of performance.  The roof decks get up to 150 (or more) degrees which basically act like a radiator shining IR down onto the floor of the attic.  Even if the air in the attic were somehow kept to 105, the IR would still be beaming down.
 
I think the money on this product is all between the living space and the solar irradiated surfaces of the house. . . the roof/attic, followed by south exposure then west then east.

 

[ano]

Got to 106 here today. One AC unit ran once for about 12 minutes in the 6pm hour, so 12 minutes during the high-rate span of noon - 7pm isn't too bad.
 
So now I know that pre-cooling works, and we move to the "warmer" time of year here, my next challenge is to figure out how to best transition into AC. 
 
While the peak electric rate doubles to $.088 /KWH (yes, I know not bad) what is bad is the peak charge that is an extra $13 per KW peak. So running two AC units at stage 2 for an hour would be the worst scenario. So my first plan is this.  Every hour starting at about 2pm I reduce the temp setting on the three main thermostats from 84 to 82 for 15 minutes. This will allow the AC to run up to 15 minutes per hour if needed rather then waiting until the temp crosses 84. 
 
As it gets hotter I need to consider lowering the hourly temp so the AC will run during more of the hours, or extending the 15 minute runtime.  If I can keep the AC on a minimum each hour during the high rates, then my rates will be the lowest. In fact, I'm thinking I need a monthly runtime and "activate temp" for each month, and use the new settings on the date the electric bill restarts. Once I extend the runtime, I might as well use that longer runtime for the whole month since I already paid for that higher peak charge. Then next month, it resets and starts again.

 

[Automate]

Got to 106 here today. One AC unit ran once for about 12 minutes in the 6pm hour, so 12 minutes during the high-rate span of noon - 7pm isn't too bad.
 
So now I know that pre-cooling works

 
Are you still setting the thermostat to 70 deg at 4 AM?  Did you ever compare your high thermal mass object temperatures to your air temperature to see if/when they get down to 70?

 

[ano]

I've settled on an initial warmer temp not to freeze us out of the house.
 
We set the bedroom AC to 74 overnight, but when we wake in the morning, it drops to 72. The other side of the house is set to 72 at 4am.  At 8am both sides drop to 70 and it stays there until 12 noon.  These temps are based on our comfort more than scientific study.
 
As for the cooling, its basically impossible to know when a "house" is cooled to a set temperature.  Even if the air was uniformly 70 degrees (which it never is) it likely takes days (not hours) for everything to be completely cooled.  The fact that the house will stay relatively cool 7 hours after stopping the cooling tells me there is lots of storage there, and as such it takes a long time time to store up.  Would I benefit to cool longer? Most likely but it is diminishing returns.

 

[apostolakisl]

What it comes down to, is decide the max and min temps you are willing to put up with.  Then predict how to spread the AC use  throughout the day to never go out of range.
 
The decision on run times would be based on a prediction of the temp, so you would need to have algorithms based on the weather forecast.  Like if it calls for 105, you might run the unit 20 minutes each hour.  If it calls for 95, you might run them 10 minutes each hour.  Like you said, though, once you go to 20 minutes per hour, there would be no need to back off of that until the next month starts.  You would thus want to start running the units early in the day, even though it is not yet that hot in the house, to protect yourself from having to run it more later in the day.
 
Exactly how you time it will depend on how the utility cuts up your usage blocks.  If each hour ran by the clock at the top of the hour, then you would want to run the compressor continuously over the top of the hour for reduced cycling.  In other words, 10 minutes per hour would mean a 20 minute run starting 10 minutes before the top of the hour.  Then you skip the next top of hour.
 
But if it is a floating clock, you'd have to do 10 minutes on, wait 50 minutes, 10 minutes on, etc.
 
Your algorithm would be more complicated because you have 2 stage units and you have two of them.  But the principle applies just the same.