Navitron Renewable Energy and Sustainability Forum

HEAT PUMPS & Geothermal Energy => Heat Pumps => Topic started by: furkin on September 10, 2006, 01:38:06 PM



Title: Heat Pump Basics please
Post by: furkin on September 10, 2006, 01:38:06 PM
Hi,
I've only just started looking at Heat Pumps (land/water) etc, & whilst there are (as usual) some great points on these pages - where can I go for the very basic information,,,, & first step ?

thanks all


Title: Re: Heat Pump Basics please
Post by: wyleu on September 10, 2006, 08:17:12 PM
I dont think anyone has done a what a heat pump is so I'll try to see what I can say.

Up here at the earth/air interface life has a certain element of variability. Call it weather if you must but in essence it's too bloody hot in summer and too bloody cold in winter. However about a metre down underneath the ground life is a little more consistant, you look around to find out what day it is and it's just , well the same as every other day cos your living in an enormous heat sink that exists at pretty much the average temperature all year round.
So if you can extract some of this heat you might be able to warm up the lousy winter and cool down the over hot summer.

But how might one do this? Well if you bury a couple of hundred metres of plastic pipe WITHOUT CONNECTORS in the garden during winter and pump salty water thro' it then the water will warm up. Not by a lot but it will warm up.
Now by the wonderful machinachions of Boyles law ( altogether now, pressure * volume = constant) if you take a lot of water warmed up by a bit and squeeze it throu' a same appatrure with a pump you can warm up a smaller volume of water to a higher temperature. So by spending 1Kilowatt of energy to drive the pump you end up with about 2-3 Kilowatts of hotter water.

Now there are some wonderful compressor type devices that will do this trick for you, but unfortunately the more actual temperature you try to run all this at the less efficient it all is ( Second law of thermodynamics for all you Flanders and Swann fans out there).

So an electric pump, pumping saline round the pipes and then pressurising  the water will increase the heat to the sort of level that is what we might call hot. Hey presto, spend a little energy get more back.

Over to you guys who know what they are talking about...


Title: Re: Heat Pump Basics please
Post by: furkin on September 10, 2006, 10:36:11 PM
Thanks wyleu,

,,,,sounds far to simple,,, even for me !

so in essence, if I dig a 3 feet deep trench in a great loop around my garden,,,, place a couple hundred feet of 50mm pipe in it,,,  then fill pipe with salty water,,, & fit a pump at one end, say with a 50mm input & 22mm output,,,, the already 'bit warmer' water will be even warmer (or even 'hot') ?
I guess that the 22mm would then be feed through a coil in a hotwater tank, as per any heating coil (of appropriate dia) ?

cheers



Title: Re: Heat Pump Basics please
Post by: Nick_W on September 12, 2006, 12:24:22 PM
A heat pump works exactly the same as your fridge. It runs a motor which makes one side warmer and the other side cooler. In other words it pumps heat away from one side to the other.

For central heating, a heat pump is the other way round to a fridge, the 'cold' side is connected to the pipes going under ground and the 'warm' side is in your house running your underfloor heating.

Going back to your question:

Clearly, the ground will only heat the water in the pipe if the ground is warmer than the water. The ground at this depth is a constant temperature (about 10C)

That doesn't sound very much but it's a big help for heat pumps: the more a heat pump runs, the colder the cold side gets and the warmer the warm side gets.

If the cold side starts to freeze, the heat pump has work harder and becomes inefficient. This is where the constant underground temperature comes in, you pump very cold water in one end of your ground loop and get warmer water when it comes back out.

So in effect, you are drawing heat out of the ground even though it appears there isn't that much heat to be had down there.

Hope this helps.