Increasing total efficiency by combined

district cooling and power with desalination

in Middle East

Track: Market Trends, Combined Power

and Water

by

Hanna Alavillamo

Wärtsilä Finland (Finland)

and

Upma Koul

Wärtsilä Dubai (UAE)

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Table of Contents

1. Executive summary ............................................................................................................ 4

2. Introduction ........................................................................................................................ 4

Smart Power Generation (SPG) ......................................................................................... 4

3. Case description and assumptions ...................................................................................... 5

4. Desalination methods for ICE power plant ........................................................................ 6

Reverse Osmosis (RO) ....................................................................................................... 6

Thermal desalination .......................................................................................................... 7

5. Absorption chiller cooling method ..................................................................................... 8

Working principle .............................................................................................................. 8

District cooling with absorption chiller in ICE power plant .............................................. 9

6. Combining chilling and desalination in one power plant ................................................. 11

Reverse osmosis desalination ........................................................................................... 11

Thermal desalination ........................................................................................................ 12

7. Conclusions ...................................................................................................................... 13

Sources: ............................................................................................................................ 14

Appendixes ....................................................................................................................... 15

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Legal disclaimer

This document is provided for informational purposes only and may not be incorporated into

any agreement. The information and conclusions in this document are based upon

calculations (including software built-in assumptions), observations, assumptions, publicly

available competitor information, and other information obtained by Wärtsilä or provided to

Wärtsilä by its customers, prospective customers or other third parties (the ”information”)

and is not intended to substitute independent evaluation. No representation or warranty of

any kind is made in respect of any such information. Wärtsilä expressly disclaims any

responsibility for, and does not guarantee, the correctness or the completeness of the

information. The calculations and assumptions included in the information do not necessarily

take into account all the factors that could be relevant.

Nothing in this document shall be construed as a guarantee or warranty of the performance

of any Wärtsilä equipment or installation or the savings or other benefits that could be

achieved by using Wärtsilä technology, equipment or installations instead of any or other

technology.

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1. Executive summary

In the northern parts of the world, electricity consumption is at the highest in the winter time

when it is cold outside and buildings must be kept warm by electricity. This is why district

heating applications are very common, especially in Northern Europe and Russia. Heating is

produced from energy (heat) that would otherwise be wasted in electricity production. In the

Middle East the situation is the opposite, as air conditioning is one of the biggest consumers

of electricity. However, also in the Middle East there is waste heat available in electricity

production, and this energy can be utilized for cooling by the use of e.g. an absorption chiller.

District cooling can easily be integrated to internal combustion engine (ICE) driven power

plants, thus increasing the total efficiency of the power plant up to >75%.

Another very crucial issue in the Middle East is the shortage of fresh water. There are several

methods available for utilising the waste heat of an ICE power plant also for desalination. In

this paper two of these methods are introduced. One option is to install a reverse osmosis

(RO) process, enabling fresh water to be produced simultaneously with electricity and district

cooling. Another method that is introduced is thermal desalination where high temperature

(HT) cooling water of the engines is used for desalination.

In this paper, district cooling and desalination possibilities in the Middle East are studied.

Also it is demonstrated how these two technologies could be combined in one power plant,

hence providing electricity, cooling and fresh water from one power plant. A case example is

an imaginary shopping mall in UAE climate.

2. Introduction

Smart Power Generation (SPG)

Electricity markets are changing all over the world. The drivers for this change vary

drastically depending on the country. One common factor in the majority of the markets is the

need to cut down greenhouse gas emissions and increase the share of renewable energy. This

has led to a dramatic growth in solar and wind power production, which in turn has started to

put a lot of pressure towards installing more flexible capacity to provide energy when the

production is not achieved by the renewables.

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According to the International Energy Agency IEA (1), internal combustion engine (ICE)

power plants are an excellent means to provide flexibility to the electricity market in order to

be able to maintain stability in the system regardless of variations in the renewable energy

production. The engines can ramp up to full load in just 5 minutes, and when no longer

needed, they can be shut down in just one minute. This can be done as many times per day as

needed, without any impact on the operation and maintenance (O&M) costs of the power

plant, and only marginal change in the efficiency. This enables the maximal penetration of

renewables into the power system, cutting down CO2 emissions and also providing dramatic

savings (2). This concept is called Smart Power Generation (SPG).

3. Case description and assumptions

In this study, an imaginary shopping mall, size 100,000 m2 is used as an example. As this is

not a real life case, only an introduction to the possible techniques that could work in this kind

of an application, assumptions have been made and also only preliminary calculations and

flow diagrams are provided.

The electricity for the shopping mall is provided by two Wärtsilä 20V34SG engines, each

having the electrical capacity of 9.7 MW at the ambient temperature 38°C. It is assumed that

possible excess electrical capacity produced by the engines can be sold to the grid and in case

not enough power is provided by the engines, the lack can be bought from the grid. This way

the plant can always run according to the cooling need instead of electricity consumption. An

estimation for the electricity consumption in the mall is roughly 215 kWh/m2.

Cooling of the shopping mall is mostly achieved by exhaust gas driven absorption chillers. In

the summer time also electrical chillers are needed to keep the temperature inside the mall on

a convenient level.

It is assumed that the fresh water requirement for this size of a shopping mall is about 11 m3/h

(2). In this study two solutions are introduced for providing the fresh water: Reverse osmosis

(RO) and thermal desalination (TD). These systems are introduced more in detail in the next

chapter.

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4. Desalination methods for ICE power plant

Reverse Osmosis (RO)

Reverse osmosis (RO) is a widely used technology for fresh water production. In RO power

plants, water is demineralized and deionized by using pressure to force it through a semi-

permeable membrane (a membrane that allows atoms or molecules of a certain size to

penetrate through).

Figure 1. Reverse osmosis principle

The required pressure for the RO depends on the salt concentration in the salt water. Up to

99+% of the dissolved salt is left to the reject stream. Also a large amount of particles,

colloids, organics, bacteria and pyrogens are removed.

The RO system can consist of one or two stages. In the case of a one-stage system, the salt

water enters the system as one stream and exits in either the reject stream or fresh water

stream. In a two-stage system, the reject of the stage one is used as feed water for the second

stage. The fresh water product streams of both stages are combined. This increases the fresh

water recovery of the RO plant. (5)

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When sea water is used as the feed for a RO plant, the system has to be a so called two-pass

system due to the high salinity of sea water. The permeate (product) from pass 1 shall be

further purified in pass 2. This is illustrated in figure 2.

Figure 2. Two-pass reverse osmosis principle

Thermal desalination

Thermal desalination techniques are based on heating salt water to a certain temperature and

then collecting the condensate (distillate) to produce fresh water. There are several methods

for thermal desalination, here only one of them, multi-stage flash distillation (MSF) is

introduced.

In this process, the distillate passes through several chambers. Each successive stage is

operated at progressively lower pressure than the one before. Before the first chamber, the

water is heated at a high pressure. When it is lead to the first chamber, the pressure is released

and the water boils rapidly. This causes sudden evaporation, “flashing”. This is continued in

the following chambers and the vapour is collected by condensing it into distillate in a heat

exchanger following each stage. The heat exchangers are cooled by the inlet water.

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Figure 3. MSF working principle

About 50% of the installed desalination capacity worldwide is provided by thermal processes,

out of which 84% is MSF. These plants are especially widely used in the Middle East. (5)

The MSF plants are typically highly reliable, which makes the technology widely used. (6)

5. Absorption chiller cooling method

In order to take the advantage of waste heat in ICE power plants especially in hot climate,

absorption chillers can be used. Typically waste heat is utilized in the form of hot water,

steam or exhaust gas.

Working principle

The absorption chiller working principle is based on three basic thermodynamic principles

(7):

Heating a liquid causes it to vaporize, cooling a gas causes it to condensate.

Vaporizing consumes energy, condensation releases it.

Lowering the pressure reduces the boiling point

Heat flow direction is from lower to higher temperature.

Basic working principle is presented in Figure 4.

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Figure 4. Working principle of an absorption chiller. For exhaust gas or steam driven chiller the COP is

1.35.

Two fluids that are easily dissolved in each other are used, a refrigerant and an absorbent.

Typically water is used as the refrigerant as it can easily change phase between liquid and

vapour. The absorbent which causes the refrigerant to boil at lower pressure and temperature,

can be e.g. ammonia or lithium bromide. Waste heat can be used as the heating source for the

absorption chiller.

District cooling with an absorption chiller in an ICE power plant

Waste heat of an ICE power plant can be utilized to provide cooling by using an absorption

chiller. The source of heat can be either hot water, steam or exhaust gas. In case hot water or

steam is used, a waste heat boiler is installed after the engine in order to use the heat of the

exhaust gas. In this study we are only focusing on a direct exhaust gas driven absorption

chiller. A basic flow diagram of such a system is shown in figure 4.

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Figure 5. An exhaust gas driven absorption chiller in an ICE power plant (simplified example)

The exhaust gas of the power plant is led to the chiller before it is led to the chimney. Exhaust

gas temperature in this case (as described in previous chapter) is 368°C, the flow being

15.3kg/s. The total electrical output is 19.4 MW, absorption cooling chiller capacity is about

12.6 MW. Please see Appendix 1 for the plant details. The total cooling requirement for this

type of a shopping mall should be calculated with adequate software and is therefore not

included in the scope of this study. We can roughly estimate that the cooling consumption

would be around 150 W/m2, leading to the consumption of roughly 15 MW. This means that

part of the required cooling would be supplied by electrical chillers. The need for chilling

naturally varies drastically throughout the year.

Another option is to have a steam driven chiller. In this case there would be a waste heat

recovery boiler after the engine to boil water with the exhaust gas energy. The steam would

then be used as the heating source for the absorption chiller. This arrangement would give the

cooling capacity of 13.5 MW. The benefit of a steam driven chiller is that the steam could

also be used for the desalination process (see chapter 5.2) and the control of these two

systems (chilling and desalination) would be easier than with separate systems. The drawback

would be the additional costs of the boiler. See appendix 2 for details of this arrangement.

The gross electrical efficiency of the power plant is in the given circumstances 44.9%. The

exhaust gas driven chiller would increase the total efficiency up to 74.3% and the steam

driven chiller up to 75.8%.

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6. Combining chilling and desalination in one power plant

Reverse osmosis desalination

In a case where reverse osmosis is used for desalination, one option is to take raw water for

the RO plant via a beach well. In a beach well, water is taken through a bed of sand from the

bottom of the sea, thus providing natural filtration and better quality of the intake water.

According to a study (8), this is a very cost-effective way to provide raw water to a RO plant,

reducing the costs by up to 17%.

Figure 6. Beach well principle

The water is taken from the beach well into the RO power plant. A modern RO plant

consumes electricity minimum a minimum 3.2 kWh/t permeate, which is provided by the ICE

power plant. The cooling of the power plant is performed by cooling towers.

A schematic drawing of an application using a beach well intake for a RO plant is presented

on figure 7.

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Figure 7. Beach well intake for a RO plant

Thermal desalination

There are several alternatives for providing heat to the thermal desalination unit. One of them

is to use engine cooling water (HT water), which gives in this example a 44 kg/s flow per

engine at a 96°C temperature. Using this water in, e.g., 10 stage MSF, the heat consumption

would be around 100kW/t distillate, leading to a a production rate of 30 t/h. In this case HT

water is cooled down to 80°C leading to around 3000kW heat input). In case a multi effect

evaporator that is based on a different technology is used, the fresh water capacity to

consumption would typically be between 10.5-16.7 m3/h depending on the unit thermal

efficiency.

It is possible to increase water production by using a booster heater for the HT water before

the desalination, which means that waste heat of the exhaust gas is used to pre-heat the HT

water.

Another option would be (as described in chapter 4) to use steam as the heating media for

thermal desalination. Steam would in this case be used also for the absorption chiller. This

would enable seamless control of the two systems (chilling and desalination), but increase the

capital costs of the plant.

In case thermal desalination is used, the intake water amount is so big that there is no sense in

using a beach well and itis not necessary for MSF technology. Instead, direct sea water intake

is used. In this case also the engine cooling could be done by raw water and hence avoid

having cooling towers.

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7. Conclusions

Internal combustion engine (ICE) power plants can provide three products, i.e. electricity,

cooling and fresh water, to large commercial buildings in the Middle East. The methods for

this are many, in this paper some of them were presented on a very general level. The first

alternative that was presented is a reverse osmosis (RO) plant which takes the inlet water

through a beach well. In this case chilling would always be done by exhaust gas driven

chillers (supported with compressor chillers) and engine cooling by cooling towers. With this

alternative, electricity consumption for desalination would be low and there would basically

be no limit for the fresh water generation.

Another alternative would be to use thermal desalination and use engine cooling water (HT

water) as the heat source for the desalination equipment. Intake water for the desalination

would be taken from a direct sea water intake, also engines could be cooled with raw water

cooling. Steam could also in this case be used as heat source for the desalination plant, in

which case the chillers could be steam driven as well, thus enabling seamless control of the

cooling and desalination systems.

Regardless of the method that is used, further investigation should be done in order to find the

optimal solution for the needs of the plant in question. Nevertheless it is obvious that it is

possible to find a very cost effective, efficient, compact and environmentally sound solution

for Middle Eastern conditions by a combination of ICEs, absorption chillers and a

desalination plant.

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Sources:

1) Energy Technology Perspective: Harnessing Electricity’s

potential, IES2014

2) Water use benchmarks, Major regional and regional shopping

centres,

http://www.watercorporation.com.au/home/business/saving-

water/case-studies/shopping-centres, accessed 20.7.2015

3) Kilmstra, Hotakainen: Smart Power Generation, 2013

4) What is Reverse Osmosis, http://puretecwater.com/what-is-

reverse-osmosis.html, accessed 20.7.2015

5) Krishna: Introduction to Desalination Technologies,

http://www.twdb.texas.gov/publications/reports/numbered_repo

rts/doc/r363/c1.pdf, accessed 20.7.2015

6) Münk: Ecological and economic analysis of seawater

desalination plants, 2008

7) Absorption cooling basics,

http://energy.gov/eere/energybasics/articles/absorption-cooling-

basics, accessed 21.7.2015

8) Schwarz: Beach well intakes improve feed-water quality,

http://www.waterworld.com/articles/wwi/print/volume-

18/issue-8/editorial-focus/beach-well-intakes-improve-feed-

water-quality.html, accessed 11.8.2015

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Appendixes

Appendix 1. Plant details when using exhaust gas driven chiller

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Appendix 2: Plant details with steam driven absorption chiller