Heat Pumps

[Stan]

There has been a lot of discussion lately about new technology heat pumps that work to much lower temperature. The truth is that a heat pump can extract heat from air all the way down to -372°F. if you use nitrogen as the transfer medium.  The criteria is the cost of getting the heat into a useful form.

Does anyone know how many BTUs of electricity it takes to produce X number of BTUs  of hot air?   Let us assume an outdoor temperature of 40°F and a desired indoor temperature of 70°F. Is there a chart or graph available that lets one do a direct BTU conversion. Obviously a heat pump is much more cost effective if electricity costs 10 cents per kwh than if it costs 20 cents per kwh.

If it takes three roof mount heat pumps to cool your bus from 100° to 70°, does it take those same three 
heat pumps running in reverse to warm your bus from 40° to 70°?

[Nick Badame Refrig/ACC]

Hi Stan,

"If it takes three roof mount heat pumps to cool your bus from 100° to 70°, does it take those same three 
heat pumps running in reverse to warm your bus from 40° to 70°"

No, for one example- in winter we don't have the enormous heat load of the sun to battle. We can not
shade ourselves from the cold with an umbrella as we do with the sun. Second, Humidity- makes the air
more dence "heavier" which retains heat longer then dry air.

Hope this helps
Nick-

[ChuckMC9]

If it takes three roof mount heat pumps to cool your bus from 100° to 70°, does it take those same three heat pumps running in reverse to warm your bus from 40° to 70°?

Takes one. One to cool it 100-70°, and one to heat it from 40-70°.

Quickly enough for me. If you want it instantaneously, well then...

But once it's there, the AC, or the HP, can practically idle. I don't think I've ever had the knob more than 55% toward whichever goal I'm trying to achieve.

(15K units, very open floorplan, and very, very rarely both on at once)

[Jerry Liebler]

Stan,
      You have several questions.  First heat pumps in the US are rated in BTU/h of heat moved and in watts consumed.  The ratio of these two numbers is called the energy efficiency ratio.  Also the conditions of indoor and outdoor temperature affect both the BTU/h actually moved and the actual energy consumption,hence the EER.  The US government has mandated energy efficiency standards for residential air conditioners but no such standards exist for RV products.  RV products are generally rated at outdoor temperatures of 80f and indoor temperatures of 80f and even with these absurd conditions have a calculated (they will never publish it directly) EER of 7 to 10.  The current standard for ductless splits requires capacity and power ratings at 95 f outside and 70f inside and under these conditions current products have an EER of 12 and over.  Extrapolating typical AC performance curves leads to my belief that RV AC and heat pumps would have EERs of 5 to 7 if rated at the residential conditions.
    Your second question is about cooling BTU requirements vs heating.   This is complicated very much by humidity or dewpoint.  It takes a lot of energy to remove moisture from air.  Here is a web site to do some of these calculations  http://www.linric.com/webpsy.htm 
To achieve  20 degrees of cooling often takes as many BTUs as 40 degrees of heating. because of the effects of humidity.
Regards
Jerry 4107 1120 

[Jerry Liebler]

Stan,
   I forgot to add in the rest of the world heat flow isn't expressed in BTU/h but watts so EER is simply watts moved over watts used.   So International EER times 3.412 is the US EER.  My 18,000 BTU/h heat pump moves 5275 watts and uses 1442 watts to do it for an international EER of 3.66 Watts moved per watt used.  Interestingly, this ratio remains the same in heating all the way down to 15 degrees F outside.  But at 15 degrees outside I'm only getting 15,000 BTU/h of heat out and the input power has dropped correspondingly
Regards
Jerry 4107 1120. 

[Stan]

Thank you Jerry. Those are useful numbers that mean more than the advertising hype. I am considering a ground source heat pump for my home and nobody would give me those numbers.  Discounting everything except actual energy cost, then the heat pump is cost effective when electricity does not cost more than 3.6 times the cost of cheaper fuel (using your example). Is this correct?

[Jerry Liebler]

Stan,
    To compare, cost of operation of,  an electrically powered heat pump to  a combustion heat pump. you must also include the efficiency of the combustion device.  Combustion efficiency of most furnaces is about 75% with the 'state of the art' about 90%.  So to factor this into your quick calculation would mean the electricity could cost 4.8 times the other fuel's cost/unit energy, with a typical furnace (like the Webasto in my bus), and not exceed the other fuel's cost of operation.  Actually the previous post slightly exaggerated the efficiency and heat output of my heat pump by neglecting the defrost cycles and just using  the specifications.  The most frequent defrost activity I've observed is defrost for 8 minutes, run for 100minutes  Which takes the average heat output down to 92.6%of  non defrost and impacts power used as well but not as much because power use drops considerably during defrost. So a better energy ratio including defrost activity would be 3 w delivered for every w used.  I think your home 'geothermal' heat pump won't suffer this loss.
Regards
Jerry 4107 1120

[Stan]

Thanks Jerry.  The reason I said comparing fuel cost only is that there are many other factors. Probably the biggest one is the difference in capital cost, plus the conversion efficiency, plus the space requirement, plus the dependability. The list goes on, but in order to get any kind of payback, you have to have a fuel cost advantage.

One of the reasons I asked the question was that on one website they recommended a ground source heat pump if you were heating with electricity, oil or propane but not natural gas. I can replace my old gas furnace with a new high efficiency one for little more than $2k. To install a ground source heat pump the estimate approaches $20k. Interest on $18k would pay my heating bill forever but the political pressure is on to go green.

[JohnEd]

Put in a regular system and use the ground water to spray the coils when you are in AC mode.  That must triple the efficiency,  Nick needed here , and it costs nothing except the cost of the pump and tubing.  With the $18K savings you could afford to replace the condenser every 4 or five years.  You should hear them rave about this in Palm Springs.  Taking an $18K hit is well beyond being described as "bowing to green pressure" .  Were I you I would LIE ABOUT MY PUMP and have enuf money to pay others to swear to it.

Good luck with this, we are watching you carefully

John

[Nick Badame Refrig/ACC]

Hi John,

Actually, "geothermal"  or "water source Heat Pumps" work very well in the mid Atlantic to southern states.

Don't think of it as spraying the coils but, a water heat exchanger. Ground water is always 55deg. so there is an enormous amount

of heat or btu's in that water. Imagine  a Heat pump system that has to dissipate the cold evaporator temps in the outside unit in heat mode.

The 55deg. water absorbes that in a heat exchanger and returns it to the ground. And vice versa in the summer time with the hot discharge gas.

Needed-
Heat pump system with a stainless steel heat exchanger
2 wells, one out, one in, or 1 well if you don't mind watering about a 1/2 acre of lawn everytime the system cycles.
or, a lake on your property to construct a continuous loop heat exchanger.
and of course a Pump.

They usually give good results in the regions indicated. As always with a Heat Pump system, you will still install a bank of electric heat as back up.

$20,000+ for a system is in the ballpark.

Good Luck
Nick-

[Jerry Liebler]

Nick,
    Is it practical to convert  my house's heat pumps to geothermal?  I have 2 rather new Carrier variable speed units using 'Puron'.  I have about a 1/2 acre pond about 300 feet away and about 20 feet lower.  Could flat plate heat exchangers between water and 'Puron' be added and some modification to the outdoor units be done?  Any way to predict the change in power used?
Regards
Jerry 4107 1120

[Nick Badame Refrig/ACC]

Hi Jerry,

Yes, it is practical.. You allready have most of the system.

Tomorrow, when I stop in my office, I will get you some very good links to check out about the pond setup.

Happy Holidays
Nick-

[JohnEd]

Nick,

I have a hunch and more about how the geothermal system or pond work.  Not that i am smart, now....just slightly conversant.  In Palm springs it is popular to add a misting system to your condenser to make it work "better".  The people with the system swear by it as reducing their electric bills and making the house cooler on those 120 degree days when many systems go marginal.  Same thing as we do with the misters on the buss' radiator.  The Geo or pond makes the AC much more efficient, I think. 

Why couldn't you simply submerge your condenser coils in the Geo or pondwater that was pumped in?  That should give you a bump in both heat and AC....right?

[Nick Badame Refrig/ACC]

Hi John,

Yes, your right! But, a funny thing happens when you mist or spray water in the open air,,, Oxidation..

And minerals will crystallize and adhere to metals. Submerged, or in a closed cell this doesn't happen as quickly.

Happy Holidays
Nick-

[gus]

As I remember from high school science there is a thing called "heat of vaporization" which explains the reason misting is so effective.