bintoito

25-02-2013, 01:54 PM

Hi all, first time poster, (relatively ;) long time reader.

I'm currently working at a University and we're involved in an energy efficiency project geared towards several Agrofood industries.

Long story short, I’m to develop a trigen system for a fish freezing industry. After analyzing their monthly electrical bills, I’ve realized that their “base-load” chiller, a Mycom VSD 200 HE (ammonia based) is responsible for roughly 40% of their yearly energy consumptions.

Initially I was hoping on going to an ammonia absorption machine that would enable me to completely meet his rated output, but after some calculations I had to abandon this idea due to the low COP –would force me to have a cogen prime-mover with an “off the scale” thermal output (the electric chiller with a reported COP of about 2 – given by the Mycom soft, consumes about 100 kWe giving 200 kW cooling; this would mean having a 400 thermal output to feed the absorption chiller…so many wastage is a big no-no these days ;)).

Now I’m looking at more “modest” prime-movers just to ease the thermal loads during peak hours.

My initial thoughts are: a trigen (LiBr-H2O based - more common and with better COP’s) system where the refrigerant – water would go in to a heat exchanger that on the water side acts like an evaporator and on the NH3 side as a condenser. I’m guessing that this would “ease” the thermal load on the evaporative cooling towers, thus lowering head pressure <-> compression ration.

I’ve inputed a 10ºC drop in the condenser temperature and indeed, the Mycom soft gives me a better COP (a 0.5 increase in COP).

But I also read that much care has to be made in lowering head pressures below certain limits, and that this is more effective on hotter months, have to put an extra pump to prevent flash gas, which in turn might provoke cavitation, etc and so on…. but what are your thoughts on this general concept? Is this doable?

Also, I would like to be more ambitious and make use of the remainder thermal output from the trigen to eventually superheat the NH3 (equivalent to raising the pressure) at the electrical chiller’s suction port. I was hoping that this would lead me to further savings as the compression ratio would be reduced, but if I increase the P or T in the suction, event though the COP goes up a bit, so does the electrical input to this chiller…this shouldn’t be happening as I’m lowering the comp. ratio so, what I’m I missing here?!

Lastly, are the values (COP, mass flows, etc) given by the Mycom software trustworthy? Because, not having NH3 flowmeters is a real nuisance in calculating COP’s….(can go with swept volume I guess, but how accurate is this method also?). For me to calculate the heat rejected to cooling towers, can I use the the liquid specific enthalpy of the ammonia on the low pressure side and the gas specific enthalpy at the high pressure side?

Like, m x (h_gas – h_liq) = rejected heat (heat from space + heat from compression) x 5 or 10% due to heat losses?!

Ps: many appologies for the big big post, but unfortunately I don’t know many people with proven hands-on experience that could advise me on possible technical barriers arising from this idea…

I'm currently working at a University and we're involved in an energy efficiency project geared towards several Agrofood industries.

Long story short, I’m to develop a trigen system for a fish freezing industry. After analyzing their monthly electrical bills, I’ve realized that their “base-load” chiller, a Mycom VSD 200 HE (ammonia based) is responsible for roughly 40% of their yearly energy consumptions.

Initially I was hoping on going to an ammonia absorption machine that would enable me to completely meet his rated output, but after some calculations I had to abandon this idea due to the low COP –would force me to have a cogen prime-mover with an “off the scale” thermal output (the electric chiller with a reported COP of about 2 – given by the Mycom soft, consumes about 100 kWe giving 200 kW cooling; this would mean having a 400 thermal output to feed the absorption chiller…so many wastage is a big no-no these days ;)).

Now I’m looking at more “modest” prime-movers just to ease the thermal loads during peak hours.

My initial thoughts are: a trigen (LiBr-H2O based - more common and with better COP’s) system where the refrigerant – water would go in to a heat exchanger that on the water side acts like an evaporator and on the NH3 side as a condenser. I’m guessing that this would “ease” the thermal load on the evaporative cooling towers, thus lowering head pressure <-> compression ration.

I’ve inputed a 10ºC drop in the condenser temperature and indeed, the Mycom soft gives me a better COP (a 0.5 increase in COP).

But I also read that much care has to be made in lowering head pressures below certain limits, and that this is more effective on hotter months, have to put an extra pump to prevent flash gas, which in turn might provoke cavitation, etc and so on…. but what are your thoughts on this general concept? Is this doable?

Also, I would like to be more ambitious and make use of the remainder thermal output from the trigen to eventually superheat the NH3 (equivalent to raising the pressure) at the electrical chiller’s suction port. I was hoping that this would lead me to further savings as the compression ratio would be reduced, but if I increase the P or T in the suction, event though the COP goes up a bit, so does the electrical input to this chiller…this shouldn’t be happening as I’m lowering the comp. ratio so, what I’m I missing here?!

Lastly, are the values (COP, mass flows, etc) given by the Mycom software trustworthy? Because, not having NH3 flowmeters is a real nuisance in calculating COP’s….(can go with swept volume I guess, but how accurate is this method also?). For me to calculate the heat rejected to cooling towers, can I use the the liquid specific enthalpy of the ammonia on the low pressure side and the gas specific enthalpy at the high pressure side?

Like, m x (h_gas – h_liq) = rejected heat (heat from space + heat from compression) x 5 or 10% due to heat losses?!

Ps: many appologies for the big big post, but unfortunately I don’t know many people with proven hands-on experience that could advise me on possible technical barriers arising from this idea…