Gas Absorption Heat Pumps 1

[Gunner77]

I am a building services project manager working on the early design stages of a building that was to use ground source heat pumps. The main contractor has discovered that they are over budget and have asked for value engineering solutions to lower the costs. We have been asked to carry out a comparison and redesign using air source heat pumps. We have since found that due to the de-frosting stage of the units and the fact that they use electrical compressors that we can not get the COP (efficiency rating) down to a level that triggers a grant payment to the end user. We have now switched our attention to gas absorption heat pumps as these have been used on other projects and their efficiency is comparable to GSHP. I have no real experience with gas absorption pumps and have been trawling the internet for explanations, schematics and case studies. I can see that the system uses a water/ammonia mixture as the transfer fluid and that there appear to be very few moving parts. I can see that heat is applied to the mixture (presumably under the natural pressure created by the expansion of heating?)and the ammonia boils and separates from the water. The ammonia then condenses against a heat transfer surface giving up its heat to a secondary heat transfer i.e. water or air for distribution to the building. The condensed ammonia then falls to a collector and is mixed with the water again in an absorption chamber and the process starts again.

Questions

1. How much heat is required to be applied to the water/ammonia mixture? Ammonia boils at -33 deg c.
1a. Does the ammonia vaporise or stay as a liquid when heated under pressure?
1b. Presumably the ammonia, if a liquid under pressure and heat, will de-pressurise and transform into a gas - this cycle then would resemble the standard refrigeration air conditioning cycle.
2. What temperature is the ammonia at the point that it transfers its heat to the water/air for distribution?
3. What creates the movement of the water/ammonia mixture in the system? I don't know if there is generally a pump or if its the movement caused by temperature change and therefore pressure.

Perhaps i have this all wrong only to be put right. I have made an appointment to meet with some manufacturers but wanted to get as much information as possible before hand.

 

[quark]

1a. That is not a relevant question. In Generator ammonia vaporises. High pressure is to increase the quantity of ammonia absorption.

1. Just enough heat (+losses) for a phase change (latent heat) and heat of dilution.

1b. Depressurisation does't happen during phase change. The idea is to condense ammonia back to liquid state at relatively higher temperatures. Yes, this is a standard refrigeration cycle as far as the refrigerant is concerned.

2. That depends upon air/liquid temperature you require. Standard absorption refrigeration systems maintain refrigeration temperature at -5 to +10C

3. There will be two pumps. One is for pumping dilute solution from absorber to generator (i.e water+ammonia) and second is to pump liquid ammonia from condenser to evaporator. Absorber to generator and evaporator to absorber flow happens by pressure difference.

 

[Gunner77]

Thank you quark, a great help. Your replies have made me realise some now obvious points in my questions.

Please find a simple schematic with some further questions.

Please bare with me i know it must be frustrating for you and i do apologise.

 

[Gunner77]

Quark,

All my silly questions have been answered by an animation that i came across, but thats' not to say that your help wast appreciated, thanks once again. I have included a link for you if you like.

http://www.yazaki-airconditioning.c...ioning/swf/080925_chiller_absorption_ani.html

 

[dcasto]

You are going to need waste heat source that is over 160F to get the grant. Or at least solar water heaters that make hot water.

For eveery BTU of cooling, you need a source that has 1.5 BTU's of heat available.

to get 1 ton of refrigeration, 12,000 BTU/hr, you'll need 18,000 BTU/hr of waste heat. The limit is about 10F delta T from Q =M * Cp * delta T 18,000= M * 1 *10 or 1800 pounds per hour or 30 pounds/min or 3.6 gallons per minute of 190F water dropping to 180F.

 

[DrRTU]

A few thoughts on lowering initial costs and payback. Use a pond or horizontal field if you have the ground area. Use plastic piping for the mains inside building. Taxes pay a very big picture on geothermal. You should be able take advantage of the 10% federal grant and accelerated deprecation of 50% the first year. Good luck on finding a simple cheap gas absorptions units that are common equipment.

http://www.dsireusa.org

should help with incentives.