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A Cogen(t) Discussion

Residential, commercial and mixed-use buildings account for a large share of the power consumption in Saudi Arabia, with HVAC systems consuming nearly 70% of a building’s total power load.

| | Jun 13, 2012 | 11:15 am
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Residential, commercial and mixed-use buildings account for a large share of the power consumption in Saudi Arabia, with HVAC systems consuming nearly 70% of a building’s total power load. In such a scenario, district cooling and cogeneration systems appear to be viable solutions to reduce the strain on the country’s power grid and, at the same time, significantly reduce greenhouse gas emissions through reduced burning of fossil fuels. According to global data, district cooling can reduce HVACR-related power consumption by 50%.

Given the energy dilemma that Saudi Arabia is facing, cogeneration could play a significant role in alleviating the problem in the Kingdom. To begin with, a myth that needs to be dispelled is that cogeneration is not a feasible option for the country. This came to light during the Climate Control Conference (C3) held in Al Khobar on October 16 and 17, 2011. The participants said that, contrary to popular belief, cogeneration was happening in several parts of Saudi Arabia, and that micro-turbines in the country were a reality.

Cogeneration made sense, they said, considering the losses experienced in the country during generation and during transmission and distribution. Substation losses in Saudi Arabia were in the region of 10%, where T&D losses came to about 15%. Against this backdrop, they said, cogeneration was much more feasible and appealing than a combined cycle regimen.

During C3 Doha

The experts at the conference asked and answered several valid questions. Their relevance to the country’s HVACR sector warrants highlighting them here:

What is the impact of doing the either centrifugal or absorption on capex, IRR and maintenance?

The experts were of the opinion that:

Wherever water and steam are involved in the region, salt and other impurities cannot be avoided as also chemicals.

If absorption is the way forward, should we opt for double-effect or single-effect?

The experts were of the opinion that:

  • It depends on the application, availability of funds and the availability of technology to produce the double effect.
  • As long as it is financially feasible, double-effect is better. If not, single-effect is better.
  • The advantage of double-effect is a better COP.
  • The disadvantage is cost and availability of technology to produce lower-vacuum vessels.
  • If Delta P is high, you need thicker steel plates to make the vessels.
  • If you have thick steel, you need a machine to roll it. Indeed, if steel becomes very thick, you might not be able to find a rolling machine that can roll very thick steel. Even if you find a special machine to do it, the cost goes up.
  • Double-effect needs three different levels of pressure vessels rather than two.

What is the capacity of absorption chillers that can be produced?

The experts were of the opinion that:

  • Knowing the electrical power output of a gas turbine (MW), you can approximate the capacity of your absorption machines by multiplying the MW power of the gas turbine by 0.7 or 0.6 to get the MW cooling of the absorption machines.
  • In the case of diesel generators, you produce about 50% of the absorption cooling from a gas turbine due to lower exhaust temperature.

Can we zero in on a formula to establish a relationship between power generation and cooling capacity in centrifugal chillers?

The experts were of the opinion that:

It is possible, but hard to pinpoint specific figures due to variations in equipment efficiency.

Trigen using diesel engine – is it ideal for small capacity? What is the maximum capacity of diesel engine?

The experts were of the opinion that:

If choosing the diesel option, the flow depends on how big the diesel engine is.

  • Trigen using diesel engine is a viable option.
  • The most crucial aspect for selecting an engine manufacturer are:
    • Good track record in Saudi Arabia
    • Strong after-sales support

If opting for the steam turbine approach, is it recommended to go for a double-stage steam turbine? And what power can be generated from a double-stage steam turbine?

The experts were of the opinion that:

High-efficiency steam turbines driving centrifugal compressors constitute a good alternative.

What is the highest COP possible?

The experts were of the opinion that:

Double-effect absorption chillers have a COP of 1.25 to 1.35.

Another expert speaks …

How should the country/sector make use of power plants to recover waste heat in power generation? Should it opt for centrifugal or absorption chillers in a bid to create a cogen regimen?

When we posed these questions to Ahmad Abushama, Business Development Director, Dalkia Utilities, had this to say:

“The industrial parties still focus on core business, and very little has been done to utilise the wasted heat. The selection between centrifugal or absorption chillers depends on feed fuel availability. If we have adequate amount of waste heat with the right temperature, absorption chiller would be, for sure, a better choice.”

When asked whether district cooling could reduce peak demand by 55% with TSE, his response was: “It depends on the end user’s profile. If DC serves different types of buildings or multi-use development, where each building meets its peak at different times, then we can achieve high peak reduction. TSE plays a key role in reducing the peak demand, which has a direct effect in reducing the plant power capacity.”


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