Saturday, 09 November 2024
The cost of water must be considered in chilled water plant efficiency, says Dan Mizesko, adding that the cost to produce a Ton/Hr is the true measure.
In my last article, I wrote regarding air-cooled new technology chillers being a strong consideration due to several factors. In this article, I will cover the cost of water in this region, specifically potable as well as treated sewage effluent (TSE) in chilled water plants, proving that when it comes to chilled water plant efficiency, kW/per ton is only half the story.
In addition, I will demonstrate just how expensive it is to generate a Ton/hr of chilled water in this region, based on what the District Cooling industry and standalone chilled water plants currently operate at regarding kW per ton and the water cost.
Dubai versus New York City
Let us understand the cost of water in the contexts of New York City and Dubai. Potable water in New York City is 0.0072 cents for one Imperial Gallon (IG), which equals AED 0.0264. Potable water in Dubai is 0.0138 US Cents for one Imperial Gallon (IG), which equals AED 0.0510.
So, the cost of potable water in Dubai is almost twice the cost of potable water in New York City.
As New York City does not have a TSE system for cooling towers, we cannot compare the cost with respect to the same in Dubai, but we can certainly compare it with the cost of potable water. Again, the cost of potable water in New York City is AED 0.0264, whereas the cost of TSE in Dubai is AED 0.013 per one IG. In other words, the cost of TSE in Dubai is half the cost of potable water in New York City. However, there are many additional costs with regard to using TSE in cooling plants as well as potential problems. I will explain both later in this article.
Now, let us be clear that although many District Cooling companies use TSE for their condenser water, there are many District Cooling plants that do not. Much in the same way, many standalone chilled water plants, located in buildings and facilities throughout the region, do not utilise TSE, preferring to use potable water.
District Cooling is now very large in the region. One provider company stated that it has over 1,500,000 tons of installed District Cooling capacity and that 25% of this capacity operates with TSE, which would translate to 375,000 tons of capacity. If we just take for argument’s sake that another 375,000 tons of capacity is air-cooled plants, that will leave 750,000 tons of installed capacity operating on fresh or potable water. Now, if we assume that these plants operate at a very aggressive 6 COC (cycles of concentration), this is quite a low amount of bleed water with condenser loop chemical water treatment – meaning less water usage than industry standards.
This would mean these 375,000 tons of cooling would use 5,809,401 IG in 24 hours. This would be in evaporation, windage/drift and bleed. The cost of LICENCE TO CHILL Dan Mizesko is Managing Partner/ President, U.S. Chiller Services International. He may be contacted at dmizesko@uscsny.com this water consumption would be AED 310,914.93. This is the exact cost per the DEWA slab calculator, including 5% VAT. That is a huge amount of fresh water consumed in a 24-hour period. The average household in Dubai consumes 110 IMPG per day, so this one-day total amount of water consumption for the 375,000 tons of water-cooled chilled water plants from just one District Cooling company operating on potable water is equal to 52,813 households in Dubai.
Another important point to note is that the blowdown water, which would be approximately 1,000,000 IMPG, is discharged into the sewer systems and must be treated and processed by the Dubai wastewater plant infrastructure. And it is important to note that this blowdown water is full of chemicals; dissolved gases, such as carbon dioxide; salts, such as carbonates and chlorides; and metal ions, such as manganese ions. These would prompt concern by any environmental entity. Now I understand all 375,000 rated tons would not be in full operation; however, I just wanted to point out the vast amount of water usage, should all this capacity be in operation.
This is why I stated in my last article that the new form of air-cooled chiller technology should really be considered by the chilled water industry at large in this region. This is also why the industry should not just report kW per ton efficiencies but total utility cost to produce a Ton/Hr of cooling. This would reflect the true plant efficiency; after all, we need to use power, water and fuel efficiently, and all should be considered when reporting efficiency. That is why cost to produce a ton/hr, in my opinion, would be the most transparent and accurate reporting method.
Each District Cooling company and standalone chilled water plant operating in the region would report their cost to produce a ton/hr. This would be an equal and fair comparison, as DEWA and Dubai Municipality would be supplying the power and water in Dubai. In Abu Dhabi, the local power and Municipality would be supplying the same, and each facility in each jurisdiction can be rated and compared to each other to highlight how well the chilled water plants are performing to each other.
A quick calculation of a District Cooling plant using potable water supplied by DEWA that operates at 0.9 kW per ton would have an actual cost to produce a Ton/Hr of AED 0.5208, with power being AED 0.4158, including 5% VAT, and potable water being AED 0.105, including 5% VAT. This is 0.011 cents per ton-hour production. Sounds low, but if you take an hourly production of 10,000 Ton/Hr, that’s AED 5,208 per hour. For a 24-hour production cycle, it would be AED 124,992.00, which would translate to an annual cost of AED 2,999,808. The main point I am trying to make is that it costs AED 0.4158 per Ton/Hr in electricity at a plant that operates at 0.9 kW/ton. It also costs AED 0.105 per Ton/hr in water cost, making the total cost to produce a Ton of cooling AED 0.5208 per Ton/ Hr. Water cost is a bit over 25% of the cost to produce a Ton of cooling, so it is substantial.
The main point is that it is obvious that a chilled water plant operating at 0.9 kW per ton and utilising potable water as condenser water makeup is not as efficient as one would think based on kW/ton, as water cost is also a consideration in the cost to generate a ton of cooling.
The real cost of TSE
Now, let us talk about TSE being used in chilled water plants. Some plants use direct TSE with chemical water treatment, in which case you will need to operate no higher than 3 COC blowdown using substantially more water versus potable water. And without question, condenser tube fouling will occur, as raw TSE has high concentrations of contaminants and high biological growth, which will lead to scale fouling and biological fouling, such as biofilm; under scale and biofilm tube pitting, which will cause increased energy consumption by the chillers well above what is found with potable water.
Let us once again look at a plant that on an average produces 10,000 ton-hours a day, and see the cost of the water – utilising raw TSE as condenser makeup water directly.
10,000 tons x 24 hours = 240,000 tons. At 3 COC operation, that would be 434.70 US Gallons per minute of water usage, broken down as follows:
The total water usage, as mentioned earlier, is 434.7 GPM. Now, 434.7 USGPM equals 361.96 IG. And 361.96 IG x 60 minutes in an hour equals 21,717.6 gallons of water per hour x 24 hours, which equals 521,222.4 of TSE water consumption x AED 0.05, which works out to AED 26,061.12 + 5% VAT, which comes to AED 27,364.176 per day. Who still thinks TSE is cheap?
Let us look at the cost per ton of production of just TSE, which is almost AED 0.115 to produce one ton of cooling. Just for the sake of an exercise, let us say that this plant’s chilled water was produced at 0.9 kW per ton. This would mean the electric cost would be AED 0.396 per ton of production, plus 5% VAT, which means the total electrical cost to produce one ton of cooling is AED 0.4158. The cooling plant reports that it runs at 0.9 efficiency kW/ton; however, the water cost is almost 28% of the total electrical cost to produce a ton. The total utility cost to produce a ton is AED 0.5308 to produce one ton of chilled water. This is the true transparent way to report efficiency of a chilled water plant, utility-wise.
We must also consider the inevitable condenser fouling that will accompany straight TSE water into the condenser water system with chemical water treatment. With potable water, mineral scaling and biological fouling are two of the most common problems associated with water-cooled systems, and the costs from these issues can be enormous. With straight TSE, the problems are exponentially larger.
There are other costs taken by the chilled water plant, such as those relating to maintenance, condenser water chemicals and labour. But those will be fractional and do not compare to the annual power and water costs.
Now, let us look at an air-cooled plant. Let us have this plant operate at 1.2 kW per ton, which is very poor performance for new-technology air[1]cooled equipment. But let us do the exercise anyway, just to prove a point. Now, with 1.2 kW per ton x AED 0.44, the cost of a kW comes to AED 0.528 per ton. Even at this poor rating of 1.2 kW per ton, it beats the water-cooled plant in cost to produce a ton of cooling. If you add the maintenance, etc., it would be less than the water-cooled plant, as the air-cooled plant would have no condenser water chemicals, no tower maintenance, no condenser pump maintenance, and many other considerations that are required with a water-cooled plant, such as tube scaling and corrosion, biofilm tube fouling, etc., all of which will decrease the efficiency and lifespan of the chiller and cooling towers.
As I stated in my last article, the new air-cooled chiller technology operates at 0.567 kW per ton, on an average. Now, if we add 20% to this for a fudge factor and for very high load days, we arrive at 0.68 kW per ton chiller. No, let us make it 0.9 kW – the same as our water-cooled plant. Remember, the new air-cooled technology is hi-lift, oil-less, magnetic[1]bearing chiller technology – the most efficient technology available, in my opinion. So even at 0.9 kW per ton, the air-cooled system produces chilled water at AED 0.4158 per ton, which is less cost when compared to the water[1]cooled plant.
Now, through considering just a few factors, we have demonstrated the huge benefits, cost-wise, to produce a ton of cooling with an air-cooled system, as opposed to a water-cooled system in this region. To be absolutely accurate, we need to consider more factors. Indeed, there are added costs in the case of water-cooled systems. What are they? Well, as pointed out previously, water-cooled chiller condensers foul! The actual rate of fouling is an efficiency loss of five per cent per month, unless the condenser heads are opened and tubes cleaned; even with this, as soon as the chiller is put back in operation, fouling occurs. This is a known industry fact.
Next, most plants owners who do use TSE understand that they need to incorporate cleaning and filtration of the TSE before it can be used in the condenser water loop. This is not only costly in terms of CAPEX but also OPEX, considering the substantial power demand made by RO plant/filtration.
At this juncture, it is prudent to examine the recommendations of the ASHRAE District Cooling Guide:
In the Arabian Gulf area, TSE is created with the following characteristics:
Manufacturers/chemical engineers should be consulted for the acceptable water quality, according to your case of design, but the following are the general minimum requirements from cooling tower manufacturers for an acceptable cost/lifecycle analysis:
To meet the above requirements, the following are the essential treatment steps (manufacturers/chemical consultants shall confirm on the required equipment, as the cost and water quality are the main design parameters in these plants):
As you can see from the above, this is an expensive proposition, not only in terms of equipment cost and installation but also in terms of cost of consumables and power. The cost of power for the TSE treatment must be taken into consideration when calculating cost to produce a ton of cooling.
A MAJOR consideration, I often hear the argument that with the high ambient temperatures in the region, air-cooled chillers are challenging. Again, let me point out the new air-cooled technology is high lift designed for these high temperatures, so the argument is no longer valid. In addition, the Gulf region in the summer is extremely humid with very high wet bulb temperatures. These wet bulb temperatures have led to chilled water plants having to resort to utilising air-cooled chillers to cool the chilled water plant condenser water loop, so the water-cooled chillers could stay in operation without shutting down on high head pressure and surge protection. Again, air-cooled chillers are utilised to keep the condenser water loop cool enough so the water-cooled chillers can stay in operation. If this does not make a huge statement in favour of air-cooled chillers in this region I guess nothing will.
In closing, I do hope the readers who are involved in the design and purchase of chilled water plants in the region will consider the new air-cooled technology for their plants, as water is limited in the region and desalination expensive. Owners of chilled water plants can reduce their overall cost to produce a ton of cooling with this newer air-cooled technology.
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