The Concept  

Conventional Fossil/Nuclear Power stations use the Steam Cycle as the Power Cycle of the plant, to provide mechanical power to a Generator, which in turn produces Electricity. Cold water is used to condense the hot water vapour in a Heat Exchanger (Condenser) at the exit of the turbine to maximise the energy conservation. This condensate can then be pumped back to the boiler, where it gets superheated, thus completing the cycle. The cooling water will gain on average 8^oK as it passes through the Condenser, removing the latent heat  required to change the phase of the working fluid from a vapour to a liquid. There are two main options possible for this cooling system, open loop and closed loop. An Open loop (once through) cooling system pumps water from either the Ocean, a river, or lake, depending on the geographical location of the Power Plant. This water then passes through the Condenser before returning to its source. Due to Environmental Legislation this water must be cooled to below a certain temperature before being discharged, this is done via Cooling Towers or Cooling Canals. A Closed loop cooling system uses freshwater in a self contained unit, where it is re-circulated back to the Condenser via Cooling Towers. The function of a Cooling Tower is to remove the heat from the coolant via convection, radiation and evaporation, and to dissipate it to the atmosphere. In general Conventional Power Plants operate with a thermal efficiency of approximately 33%, the bulk of the remaining 66% is lost in the Condenser, for example a Nuclear Plant that produces 1000MW’s of Power will discharge 2000MW’s as heat loss. This is another huge untapped thermal energy tank, but due to the low temperatures involved and often isolated geographical location of the Plants, this heat cannot be used for anything useful, such as district heating. However, there may be a solution! . As you will be aware from reading other pages in this web site, Ocean Thermal Energy Conversion (OTEC) utilises a temperature gradient of 22^oK between warm surface water and cold deep ocean water in tropical regions to produce electricity. The idea that we are developing in this project examines the thermodynamic feasibility of using the thermal discharge discussed above, as the warm water source for an OTEC plant. In essence OTEC could be acting as the bottom cycle of a Conventional Power Plant, providing additional power, increasing the overall thermal efficiency, and as an integrated cooling system to condense the working fluid of the Steam Cycle. This broadens the scope of potential sites suitable for OTEC, as thermal Power Stations exist all over the Globe. There could be a very positive impact on the length and depth of the cold water pipe required to provide cold water to the OTEC cycle condenser. For example, in colder climates the surface of the ocean is at a much lower temperature, thus the temperature required could  potentially be found at higher depths. Furthermore, in warm climates the cooling water used in the Steam cycle is at a higher temperature due to higher ambient temperatures, therefore higher temperatures of cold water can be used to give the  22 ^oK difference needed for OTEC operation. This means that the economics of OTEC installation may be improved by the huge reduction in the length and Engineering complexities of sourcing the cold water at much higher depths, and in turn benefit the commercialisation of this technology. In addition to this, the Power required for the warm water pump can be eliminated from the calculation of the Net Power produced by the OTEC cycle, due to this pump already being required by the Conventional Power Plant to pump the water to the Cooling Towers/ Cooling Canals. Hence the efficiency of the OTEC plant will be improved. In theory with this integration of OTEC, the Cooling Towers / Canals could be eliminated from the design of Conventional Power Plants, because there could be a sufficient temperature drop across the OTEC cycle. This water can then be re-circulated in a closed loop or discharged back to the source in a once through system. Elimination of the Cooling Towers/Canals would reduce the capital cost of the Conventional Power Plant, but on the other hand new capital costs would be introduced by the installation of the OTEC plant. The balance between these two capital costs and the revenue raised from the additional Power produced by the OTEC plant is another project within itself and is not considered in any depth within this study. The study that we have carried out involves the integration of the OTEC Ammonia Closed Cycle with a National Thermal Power Corporation, Combined Cycle Gas fired plant in India, and Longannet Power Station in Scotland UK. The source data from India was gained through one of our team members who is a current employee of this particular plant, and the data from Longannet Power Station was gained during a site visit as part of the MSc course syllabus. The Ammonia closed cycle was chosen for its simplicity and suitability to electricity production, as opposed to the Open cycle, that produces a combination of electricity and fresh water. Due to time and facility constraints a practical laboratory experiment was decided unsuitable, hence a simulation model was created on Excel that enabled us to carry out theoretical thermodynamic experiments and gain useful and interesting results. What we aim to show you is the increase in Power and thermal efficiency that can be achieved with OTEC-Conventional Power plant integration, in both warm and cold climates, proceeding with a results analysis and discussion of the potential feasibility of this idea, with reference to the assumptions we have made. TOP  

The Systems

The OTEC-Ammonia Closed Cycle

Warm water is pumped to the Evaporator where the heat is transferred through a series of pipes to the working fluid – Ammonia. During normal applications of OTEC this warm water is taken from the surface of certain areas of the tropical oceans, but in this application the warm water is drawn from the condenser of a Conventional Power Plant. The operating conditions of the steam cycle, and the original temperature of this water at the source (for open loop cooling systems), i.e. the ocean/river/lake, sets the temperature of this water, or ambient temperature in the case of closed loop systems. Cold water is pumped from the ocean through a pipe at depths where a sufficient temperature exists to give the 22^oK temperature difference between the cold and the warm water source, for example in India, a 40^oC warm water source from a Power Plant will require a maximum of 18 ^oC cold water, although 15 ^oC would be more appropriate to allow for fluctuations in the warm water temperature. This cold water is passed through a condenser where heat from the Ammonia is transferred to it, hence raising the temperature of the water. With respect to the working fluid  in this cycle, the Ammonia exists as a liquid at point 1 (ref. to schematic) and as this liquid passes through the evaporator at constant pressure it changes phase to a saturated vapour (point 2). This is due to the heat transfer from the warm water through the heat exchanging pipe network within the evaporator. The Ammonia in the saturated vapour state now has enough energy to drive a turbine specifically designed to work with this fluid. As this energy is converted to mechanical power in the turbine the fluid expands and looses energy as its pressure decreases(points 2 to 3). At the exit of the turbine the Ammonia fluid exists as a wet vapour at low pressure. To conserve as much of this remaining energy as possible the fluid passes through the condenser and looses enough latent heat to change the phase from a wet vapour to a saturated liquid at constant pressure(point 4). This occurs due to the cooling effect created by the cold water pumped from the ocean, in the same way as the ammonia was vaporised by the warm water through the heat exchange mechanism in the evaporator. To complete the cycle, the ammonia now in a liquid state needs to be pumped back to the evaporator and to the pressure required for vaporisation within the evaporator. The operating pressures within the evaporator and condenser are the saturation pressures at the respective evaporation and condensing temperatures. These temperatures are highly dependent on the temperatures of the warm and cold water sources and their respective mass flow rates. This relationship is discussed in more detail later in this page. The mechanical power produced in the turbine is then transferred to electrical energy in a generator. The output of this generator is the Gross power produced by the cycle. Some of this power is then used to drive the cold water pump and the ammonia pump, the remaining power is known as the Net power produced by the cycle. The Net thermal efficiency is defined as the ratio of the Net Power with the heat supplied to the cycle from the warm water at the evaporator stage.  TOP

The Conventional Power Plant Steam Cycle

     

The above diagram depicts the Combined Cycle Gas and Steam Plant in India, this is common to many power stations around the world. Alternatively the Gas Cycle will be replace by a Coal/Oil Furnace or Nuclear Reactor to provide the heat to the Steam Cycle. The exhaust gases from the Gas Turbine Plant enter the Heat Recovery Steam Generator where feed water, pumped from a de-aerator by means of feed pumps, is turned into steam, superheated and expands in a 2 stage steam turbine rated at 130MW.  The expanded steam enters the condenser where circulating water condenses the steam into saturated water at vacuum pressures.The condensate is then pumped back to the de-aerator and the cycle repeats.TOP

Practical Limitations of The Steam Cycle

The purpose of the OTEC integration with Steam Cycle is not only to boost up the overall power output but also the overall cycle efficiencies. This is only possible if the design limitations of the Steam Cycle are honoured. If these limitations are exceeded it will result in a overall reduced output due to the fact that the reduction in steam cycle output would exceed the additional OTEC output.

The limiting design factors of the Steam Cycle when concerned with OTEC integration are

Condenser back pressure(vacuum) and its saturation temperature which are fixed by design  for optimum steam cycle output.

The difference between the temperatures of condensate and the cooling water leaving the condenser known as Terminal Temperature Difference (TTD), which has a minimum limit that is fixed by design for maximum steam cycle output. The TTD is the minimum difference possible between the cooling water exit temperature and the steam condensing temperature, for optimum performance of the cycle.TOPThe mass flow of the condenser cooling water pump has a maximum limit which is fixed by the pump power rating.

The Model

Below still pictures of the workbooks used in this model are shown. For the purpose of actually using and developing the our mathematical model the hyperlink here allows the workbook to be downloaded or opened in excel. Open work books

 

 

Above you can see a still shot of the experimentation tool that we have developed. It consists of the basic OTEC-Ammonia closed cycle, the governing thermodynamic equations, the corresponding assumptions, interpolation calculators, and the integrated system with a conventional power plant, where the OTEC system replaces the Cooling Towers/Canals. In Excel this tool works within four sheets, and these sheets are linked via Excel formulae. The operation of the OTEC cycle can be altered in the calculator section (sheet 3) to suit the individual assumptions of the experiment to be carried out. Once the function of the cycle has been designed, the experiments can be conducted on the schematic diagram itself either for the OTEC only cycle or for the combined system. In addition, a calculator for the Steam cycle has also been developed (sheet 4), this allows the user to make changes to the operation of the Steam Condenser when doing more advanced experiments.TOP

Experiments using this workbook can be carried out using data for any Conventional Power Station for the combined system, and for any suitable warm/cold water source for a stand alone OTEC system. An electronic copy can be made available for further development through the ‘Energy Systems Research Unit, University of Strathclyde or by downloading from this site below the Model title. TOP

The Assumptions

The following list identifies the simplified assumptions used within the current Workbook:

For OTEC-Ammonia Cycle

1.Steady State conditions are assumed throughout this analysis.

2.The Power required for the warm water pump is not included in the calculation for the Net Power output.

3.There is no temperature change of Ammonia across both the Evaporator and the Condenser, only a phase change.

4.Ammonia exits the Evaporator as a saturated vapour with no degree of Superheat.

5.Ammonia exits the Condenser as a saturated liquid with no under cooling.

6.The mass flow of the warm water source is limited by the size of the existing pump in the steam cycle plant. In the case of the power plant in India the pump is rated at a maximum of  7222 kg/s.

7.The dimensional limitations of the Evaporator and Condenser are not considered.

8.The Turbine operates with an expansion efficiency of 0.85, which is attainable in current turbine technology.

9.The change in temperature across the Ammonia pump is taken as zero.

10.Pump efficiencies are taken as 0.7.For the Steam Cycle at the Indian Power Plant Steady State conditions assumed throughout this analysis.

For the steam cycle at the Indian Power Plant:

11.Steady state conditions assumed throughout this analysis

12.Design values of Steam Cycle in the Combined Cycle ( Gas Turbine+Steam Turbine) Indian Power Plant are based on ISO conditions on Gas Turbine Plant. (ISO- Amb Temp-28^oC, RH-60%, Freq-50Hz) class=MsoBodyText>

13.A simple steam cycle considered for analytical purposes.

14.The power output of the gas turbine is omitted for calculating efficiencies.

15.Also feed heating, de-aerating and condensate pumping power not considered. Steam flow increased by 30 T/hr from design flow to compensate for these omissions to get the design output.

16.Isentropic efficiency of Steam Turbine assumed at 85%.

17.All pump efficiencies within the Steam cycle are assumed to be 85%.

Characteristic Assumptions

	Realistic	Optimistic	Extreme
Twwi  oC   max	40	40	44
Twwo  oC	Twwi- (Ttot/8)	Twwi-8	Twwi-8
TE    oC	Twwo-2	Twwo	Twwo-2
mWW kg/s   max	6111	6111	7222
Tcwi   oC	15	12	4
Tcwo  oC	TC-2	TC -1	TC-2
TC  oC	optimise	optimise	optimise

 

Source Data

The following table displays the data we have obtained to assist with the experiments:TOP

Power Plant in India

Gas Turbine Plant
Load	115 MW
Exhaust Gas Temp	555 oC
Fuel Air Ratio	1:60
Steam Turbine Plant
Position in cycle	Temp oC	Pressure, bar	State
Turbine inlet	520	77	Superheated
Condenser inlet (steam)	45	0.08	Wet Steam
Condenser outlet	45	0.08	Saturated liquid
Boiler inlet	135	80	liquid
Mass flow of steam : 400 T/hr
Coolant flow rate :22000 T/hr, 26000 T/hr maximum
Terminal Temp difference	4 oC

Ambient temperature changes in a typical year at the Power plant in India

Month	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Temp oC	18	20	22	24	26	28	30	32	34	36	24	22

Data Obtained from Longannet Power Station, Fife, Scotland, UK

Turbine Inlet (steam):	Temperature	561	oC
	Pressure	125	bar
	Mass flow	362	Kg/s
Generator Load:	Net Power	515	MW
Condenser:	Pressure	40	mbar
	Temperature	29	oC
Coolant water:	Inlet temperature	7.4	oC
	Outlet temperature	14.3	oC

Results Analysis

Set 1: Analysis of Steam-OTEC Ammonia cycle integration, with the plant design of the NTPC Combined Cycle Gas Turbine Plant, in Kayamkulam, India.

At different times of the year local ambient temperatures will change depending on the natural climatic seasonal changes. This has a direct effect on the temperatures of the warm water supplied to the OTEC plant since the warmer the water entering the condenser in the steam plant the warmer the water leaving the condenser in the steam plant andon to the otec plant. We have assumed that a temperature difference of 8^oK is maintained across the Steam Cycle Condenser with respect to the inlet cooling water temperature, which is in turn governed by the local ambient temperature. This means that as ambient temperature changes the warm water source for the OTEC plant will vary accordingly. Experiments to show how this effects the Net power from the OTEC plant, found that as the ambient temperature increases the OTEC Net Power also increases to a maximum level defined by the terminal temperature difference of the Steam Cycle Condenser. This is shown on chart 2. The thermal efficiency also changes with a similar trend. To show how this effects the yearly energy production from the OTEC plant the average Net Power for each month of a typical year was plotted (chart 3). This was based on the average monthly ambient temperatures at the site of the plant in India.  The average Net power over a typical year ranges from a realistic value of 0.71 MW, to an optimistic value of 1.95 MW, and a total yearly energy production of 6.23 GWhr (realistic) ranging to 17 GWhr (optimistic). This shows a substantial reduction in the output of approximately a third, when realistic heat exchanger assumptions are introduced. However, 6.23 GWhr is still a huge amount of energy that would otherwise have been wasted. In Britain, based on a market value of 3 pence per unit of electricity, this energy could generate £190,000 of revenue. The Net Power output in the winter season will be reduced to zero, as the ambient temperature decreases. For the realistic and optimistic cases during the winter months, the ambient temperature is too low to provide sufficient warm water temperatures for OTEC Net Power production. During these months the OTEC plant will now require an input of external power from the main plant to remain operational as the cooling mechanism for the steam cycle. This would not occur in areas where a sufficiently high ambient temperature was maintained throughout the year. TOP

The OTEC-Ammonia plant output when integrated with the steam cycle output can be seen in chart 4 for the same variations in ambient temperature, and chart 5 shows the yearly Net Power fluctuations. These are quite obviously akin to charts 2 and 3 already discussed. Now we can see the overall effect of the OTEC integration, when compared to the steam cycle operating as a independent system. Clearly, the combined Net Power is simply the sum of the two power supplies, this combination increases the Net Thermal Efficiency in  a slightly different way as this must be calculated using the ratio of the heat supplied at the heat recovery boiler of the Gas-Steam cycle, to the combined Steam-OTEC Net Power output. As we would expect the trend lines for this combined thermal efficiency follow an identical pattern to that of the combined Net Power and are not shown here.TOP

Case	% increase in Eth
Realistic	0.42
Optimistic	1.12
Extreme	2.82

This table shows the maximum increase possible for Combined thermal efficiency, with respect to an optimum efficiency of 34.38% of the Steam Cycle when operating independently. These increases may appear to be insignificant, however slight improvements in the thermal efficiency of the Steam cycle working at optimum conditions at this level could be seen as a worthy achievement, although this will probably not be economically viable based on the current economic situation with OTEC technology.TOP

The results of a more advanced experiment showing how changing the Terminal Temperature Difference (TTD) of the Steam Cycle effects the Net Power output, can be seen in chart 6. The optimistic case for the OTEC plant was chosen here. There is a significant drop in the power output when the TTD is changed from 8^oK to 4^oK. This shows that the Steam Cycle operating parameters should not be changed beyond the optimum limits of the TTD in the condenser. In the next set of experiments with Longannet Power Station, the TTD has been set at the limit as a requirement to gain a sufficient exit temperature from the Steam Cycle condenser for OTEC operation.TOP

Set 2 Analysis of Steam-OTEC Ammonia cycle integration with the plant design of Longannet Power Station, Scotland UK.

For the OTEC-Ammonia Cycle only

	Legend	Units	Different OTEC assumption cases			Steam Cycle
			Realistic	Optimistic	Extreme
Indian Power Plant
Efficiency	Eth max	%	1.95	2.05	4.27	34.38
Net Power	WNT	MW	1.56	4.2	10.4	128
Yearly Ave Power Output		MW	0.71	1.95	7.43	128
Yearly Energy Production		GWhr	6.23	17.08	65.09	1120.4
Combined cycle	Eth max	%	34.8	35.5	37.2
Combined cycle	WNT max	MW	129.5	132.1	138.4
Cold water inlet	Tcwi	oC	15	12	4
Cold water outlet	Tcwi	oC	23	17	12
Longannet Power Plant
Efficiency	Eth max	%	0.63	0.92		33
Net Power	WNT	MW	4.2	10.8		515
Combined cycle	Eth max	%	33.27	33.69
Combined cycle	WNT max	MW	519.2	525.8
Cold water inlet	Tcwi	oC	4	4
Cold water outlet	Tcwi	oC	14	15

Environmental Impact of The Cold Water Discharge

The following table shows the monthly average surface water temperatures at the respective countries:

This hypothetical thermodynamic study considers replacing the existing cooling system of conventional power plants both in India and in Scotland, with an OTEC-Ammonia cycle as a closed loop cooling system. As we have discussed already an additional cold water source is required. This holistic system assumes that the design of the Power Plants we have investigated can be adapted to accommodate the addition of the OTEC cycle when considering a new build Power station at a location with economic access to the respective cold ocean water required. The following table shows how each of the discharge temperatures compares with the mean surface water temperature for each case. TOP

	Cold water Inlet temp.	Cold water Outlet temp. The discharge	Mean surface Water temp.	Surface minus discharge temp.	Mass flow rate of the discharge
	oC	oC	oC	oK	Tonnes/sec
India
Realistic	15	23.75	17.75	+5.25	5
Optimistic	12	17	17.75	-0.75	9.5
Extreme	4	9.5	17.75	-8.25	10
Scotland
Realistic	4	14	10.67	+3.33	26
Optimistic	4	15	10.67	+4.33	39

With respect to the realistic cases, discharging water to the surface of the nearby ocean by up to 5.25 degrees above the temperature of the surface water at these flow rates, would probably not have any serious environmental impact. Although to minimise the thermal impact completely the discharge water would need to pass through Cooling Canals to reduce the temperature to match that of the surface water temperature. This would be enforced by Environmental Legislation if the cold water source was from a river, lake,or estuary as the eco-structure is smaller and more sensitive to temperature fluctuations, when compared to the dilution of the discharge with vast volumes of water in the ocean.  In these circumstances the discharge water temperature is required to be equal to the temperature of the intake water. For example, at Longannet Power station the existing Cooling system pumps water from the River Forth at 7.4 ^oC, and the exit temperature from the actual Plant is at 14.3 ^oC. This is then cooled in a Cooling Canal back to a temperature of 7.4 ^oC, as a requirement of Environmental Legislation enforced by the Scottish Environmental Protection Act (SEPA). If regulations regarding the temperature limits of thermal discharge water were not adhered to, then there would be an impact on the local aquatic environment, mainly concerning marine organisms and the reduction in dissolved oxygen that effects fish respiration. Like most Environmental Impact assessments this is entirely site specific, and due to time constraints, we have not investigated the location of suitable sites or an in-depth environmental analysis within this study.TOP

Conclusions

An Engineering design, thermodynamic analysis, optimisation and simulation of an OTEC power plant integration with two separate conventional fossil fuelled power plants was carried out in this project. This involved using the OTEC plant as the cooling system, replacing the existing towers or canals, and in turn achieving additional power and an overall increase in the thermal efficiency of the Power plant.

A plant simulation was further carried out to study the seasonal variation of ambient temperature for the scenario in India. A large fluctuation was discovered, although 6.23 GWhr will theoretically be produced in a typical year, holistically gaining £190,000 of revenue if traded in the British electricity market. Also if the nominal Net Power produced by the OTEC plant when integrated with the Longannet Power Station, could be maintained throughout a year, then 36.8 GWhr of energy could be produced gaining £1.1million on the British Electricity market. These figures have been produced based on a total simplification of the system and with steady state conditions. If transient conditions, technical, and geographic limitations were involved then the picture would look very different, but the fact remains that we have shown the potential for this idea in the future. We also found that no additional Environmental impact would occur regarding the discharge of the cold water required for OTEC cooling.

Further Work

The hypothetical experiments conducted as part of this study have several limitations.  Some of them have already been enlisted as part of the assumptions, for example the design elements of the Heat Exchangers (OTEC Evaporator and Condenser) such as surface area, overall heat transfer coefficients, number of tubes and size. Also important aspects related to cold water piping such as length and diameter were also omitted. Detailed design of pumping systems (ammonia and cold water) also has been excluded in the present analysis. It would be quite appropriate to include all these aspects in the next part of the study, which could involve an in-depth design of these individual process elements. Then compare these design outcomes with the products already existing in the market.  This would pave the way for an economical analysis followed by economic optimisation. Hence the future work should focus on translating the hypothetical model into a practical reality involving not only the detailed design aspects but also the economical optimisation.