Advice required on suitable design for 100USRT system where electricity cost is high

[qna]

First time poster. This is a big post and I know I am asking a lot as a newbie. But you never know unless you ask right? I came across some similar topics in the forums and was hoping I could state my HVAC design problem and get some advice.

We have a building cooling requirement of 100USRT (approx). It is an 11 storey building and HVAC equipment would likely be stored on the roof/terrace at ~31m/100ft height. Electricity is generated by diesel and purchased from the local utility at 45c/kWh. Diesel can be purchased at around $1.05/litre=$4/gallon. LPG primarily used for cooking is $81 for a 45kg cylinder.

Typical solution here: VRV/VRF, such as Daikin VRV III, with COP 4.3. To my understanding 1 ton of cooling (1USRT=3.5kW) will require (3.5/4.3=0.814kW input). Which will cost (0.45*0.814=0.37c/USRT).

The HVAC problem I am looking at is to find an alternative solution which can lower the operating cost, while decreasing or keeping the same the environmental impact of diesel generated electricity @814W/USRT.

I would very much appreciate your ideas on what I could do as an alternative to the VRV type solution. Cooling only is required.

I have also given this some thought (with no expertise in the matter) and have come up with one option. I would also appreciate your assistance and advice on the solution below as well. Possible option:

The VRV cooling above is ultimately fueled by diesel through the following cycle: diesel-> diesel generator (losses) -> inefficient electrical distribution grid (losses) -> VRV (losses) -> cooling.

I was wondering if a more efficient way would be to take the same diesel to a direct fired absorption chiller: diesel -> diesel boiler (losses) -> LiBr absorption chiller (losses) -> cooling

I have done some "google" research on the above and found that there is for example a 99USRT system that uses 20.3kg/hour of light oil (which I gather is diesel) and assuming a density of 0.883kg/litre, means it uses 24 litres of diesel per hour. Including a baseload system requirement of 4.4kW, I calculate this system could produce 1USRT@0.27c ((24*1.05+4.4*0.45)/99=0.27).

HOWEVER: As I understand it, I also need to provide cooling water from a cooling tower to this system. If I understand what I am reading it needs to take 38 deg water and reduce it back to 32 deg. I cant see how I could do this and keep the cost under the 37c/USRT that is my baseline minimum.

One possibility that I can see is that we are on an island with a thin water table. Just 2m below the surface is sea water. I can get approval to dig a borehole and get access to sea water which would have a temperature of 26-28degC. I could pump this water through a single pass and then pipe it back into a second well at the same depth, possibly around 20m apart. Or I could submerge some kind of geothermal pipe and run my cooling water loop through this.

Can I do this? Surely, if this idea makes sense, others would be doing it. Would really appreciate advice on this and also the reasons why this is a bad idea.

Many thanks for your valuable time.

 

[willard3]

Read forum policies, which begin as follows:

>>>These fora should not be used to bypass your own in-depth research on the issues that affect you, nor is it intended to be a substitute for appropriate professional assistance within your field or geographical region.

You need to hire an Engineer familiar with this. Tell your boss that you don't know enough to design HVAC.

 

[Drazen]

i don't understand that such large building and such a demand would be left to google research and tips forum?!

how can any reasonable entrepreneur live such large amount of money to lottery and guesswork? you can be smart and wit, but smart and wit people are still not allowed to for instance, work as surgeons or pilots without have proper education and experience.

that might not be so dangerous to human life but it is very dangerous to client's wallet.

absorption chillers generally have quite low cop to be feasible in most of applications, but there is much more to do that simply using some simple cop formula, you should make some energy modelling to obtain any reasonable result.

 

[317069]

HOWEVER: As I understand it, I also need to provide cooling water from a cooling tower to this system. If I understand what I am reading it needs to take 38 deg water and reduce it back to 32 deg. I cant see how I could do this and keep the cost under the 37c/USRT that is my baseline minimum.
can you explain it in more details.

 

[ProcessHVAC]

If you are designing an hvac system from scratch and don't know much about it, you can come up with something that "sort of works", equipment suppliers can help with sizing selection, etc. and chances are you might stay out of trouble. However, if you client hired to you optimize, there is a benchmark and whatever capital cost your design incurs you are on the hook for an energy efficiency/performance improvement and could get easily get you in trouble.

 

[EnergyProfessional]

the advantage of diesel-generator--> electricity --> cooling is that under part load you can use that electricity for other operations of the building. those absorption chillers likely are not good under part load and typically eat energy like crazy, only used when you have waste heat or very cheap heat, which isn't diesel.

You should do a detailed energy simulation with Trace etc.

the "geothermal" idea is not bad if you are allowed... assuming you have enough seawater.

and yes, you should hire someone familiar with designing HVAC systems.

 

[qna]

Thanks to all for your comments. The general theme appears to be get expert advice. Yes, I will. But where I am, there are limited experts. Perhaps I understated that the ONLY HVAC system that can be designed locally appears to be VRV/VRF or split systems. I have had the system "designed" as such and it turns out to be approximately a 100ton system. They do not have experience in other systems. I would need to seek out somebody to design the alternative system. I was hoping to get some ideas on which solution suits my set of circumstances and then I could find a suitable designer and approach them. Again, my apologies to those who disapprove.

@317069. Ok, I jumped a bit in my logic there as the post was already too long. I was saying that powering a cooling tower @45c/kWh in addition to other costs would likely increase my calculated operating costs of 27c/USRT to above 37c/USRT (it leaves 10c/USRT x 100USRT=$10, $10/0.45($/kWh)=22kW total allowance to run cooling tower and extra pumps). For 100tons or 350kW of cooling, I guessed any savings would be eaten up. Hence, I was wondering if I could pump sea water in one-pass or submerge a geothermal loop in the sea water and run my fresh cooling water through this. I do have an extra 30m to lift the cooling water due to building height. There are definitely no locals that can help me design and determine whether this is a good idea, hence I have posted to the WWW.

@HerrKaLeun: "absorption chillers likely are not good under part load and typically eat energy like crazy" Thank you. Very useful information. Do you mean during load changes or say operating steady at 50% load. I did see a load chart vs input, so I guess I could find that and look at loss as a percentage. Regarding the energy simulation. I am pretty sure that is not done here. It is mostly about sizing HVAC according to room volume. I will be sure to inquire about that if we decide to proceed with an alternate design.

 

[EnergyProfessional]

I don't have actual experience with absorption-chillers, but this is what I imagine. What I know is that most manufacturers abandoned them for lack of demand and they only were economical where heat was cheap or you had waste-heat.

What I also know, they require quite some maintenance and not sure what sizes are available, those are large units.

Regarding energy simulation, if you want to evaluate options, energy simulation is required. One can't just "guess" what is economical. Obviously maintenance, longevity etc. play a role too. Energy is just part of the operating cost.

 

[MintJulep]

While the ground water idea is interesting I think it's not viable because of high initial costs and pumping costs.

You will either need to pump salty water to roof level - with all of the associated special materials for piping.

Or you need an intermediate loop with a salt water to regular water exchanger at ground level.

Every exchanger will cut down on the effective temperature. So if you start with salt water at 28C your intermediate loop might be 30C so you condenser is operating at 32 or 35 or so constantly.

Moving a bunch of water in two loops is not cheap. Will you generate your own electricity to run those pumps? Or will they be diesel directly?

 

[chicopee]

What type of an occupancy do you expect in that 11 story building and where is the island located?

 

[qna]

We will abort the idea and go for a VRV/VRF system. Too many unknowns for such a large project. I think if the idea had any merit, it would have been done already.