Exploration of an ORC closed ammonia cycle

Exploration of an OTEC closed ORC ammonia cycle

Introduction
Loading a model
Model settings
Cycle plot in the (h, ln(P)) chart
Fluid change
Conclusion

Introduction

Organic Rankine cycles are variants of the water vapor cycles, which are used when the hot source from which one wishes to produce mechanical power is at low or medium temperature, or when the installed power is low and steam installations are no longer economical.

When the temperature of the hot source drops, or that the installed power decreases, typically below ten MW, the performance of steam vapor cycles deteriorates, and it becomes preferable to use other thermodynamic fluids.

Since many of these are organic in nature, it is customary to qualify these organic cycles, but other types of fluids, such as ammonia or carbon dioxide, can be used.

Here we present an example of an ORC cycle intended to generate electricity from the thermal gradient of the oceans.

OTEC stands for Ocean Thermal Energy Conversion; its equivalent in French is Energie Thermique des Mers (ETM)

OTEC cycles are designed to generate electricity in warm tropical waters using the temperature difference between water at the surface (26-28 °C) and in depth (4-6 ° C), from 1000 m as shown by this figure.

Closed cycles use hot water at about 27 °C to evaporate a liquid that boils at a very low temperature, such as ammonia or an organic fluid. The vapor produced drives a turbine, then is condensed by heat exchange with cold water at about 4 °C from deeper layers of the ocean.

In all cases, the need to convey very high water flow rates and pump cold water at great depth induces significant auxiliary consumption. Optimization of an OTEC cycle is imperative to take into account those values.

The thermodynamic cycle is similar to the one we have studied for steam plants except that the working fluid is ammonia. The sizing of heat exchangers is of course even more critical given the very small temperature difference between hot and cold sources. Pinch values should be as low as possible while remaining realistic.

Closed OTEC cycle diagram

At point 1 at the condenser outlet, the working fluid is in the liquid state.

The pump compresses the ammonia to 9 bar.

The steam generator is a triple exchanger ensuring heating in the liquid state, vaporization and a slight superheating of the ammonia.

The pump and the turbine can be assumed to be adiabatic. As for the steam generator and the condenser, we can at first guess that they are isobaric.

Glossary

Adiabatic

Evolution without heat exchange with the surroundings

Reversible adiabatic

Adiabatic evolution without heat exchange with the surroundings and without friction losses

Heat

Transfer of thermal energy from one system to another when there is a temperature difference between them

Latent heat of change of state

Energy that must be supplied or withdrawn when a phase change takes place

Coefficient of performance (COP)

Generalization for the refrigeration cycles of the concept of efficiency

Cogeneration

Combined production of thermal energy and mechanical energy or electricity

Non stoichiometric combustion

Combustion with excess or lack of air.

Stoichiometric combustion

Combustion without excess or lack of air where all available oxygen is completely consumed.

Hermetic compressor

Compressor whose engine is directly cooled and lubricated by the thermodynamic fluid, which eliminates the need for oil

Scroll compressor

Compressor formed of two cylindrical spirals, one fixed, the other mobile, of identical shape, which roll by sliding one on the other, enclosing gas pockets of variable volume

Condensation

Evolution of a substance from the gaseous to the liquid state

Counter-flow

Thermodynamically efficient exchanger flow configuration

Composite curves

Set of two curves representing on the one hand the cumulative energy availability of a heat exchanger network as a function of temperature, and on the other hand, that of energy needs.

Cycle

Series of processes which bring a fluid to its initial state

Combined cycle

Integration into a single unit of mechanical power production of two complementary technologies in terms of temperature level, generally gas turbines and steam plants.

Brayton cycle

Motor cycle of a gas turbine composed of isentropic compression, isobaric heating, isentropic expansion, and isobaric cooling

Reverse Brayton cycle

Refrigeration cycle composed of isentropic compression, isobaric cooling, isentropic expansion, and isobaric heating

Carnot cycle

Motor cycle composed of isentropic compression, isothermal heating, isentropic expansion, and isothermal cooling

Hirn cycle

Rankine cycle with superheating

Rankine cycle

Motor cycle of a steam plant composed of isentropic compression, isobaric heating in two stages (heating in the liquid state, vaporization), isentropic expansion, and isobaric cooling

Motor cycle

Cycle converting heat into work

Refrigeration cycle

Cycle allowing to extract heat at low temperature or to raise the temperature level of a fluid thanks to a supply of mechanical energy

(h, ln(P)) chart

Thermodynamic chart with enthalpy on the abscissa and pressure on the ordinate, generally with a logarithmic scale.

Entropic chart

Thermodynamic chart with entropy on the abscissa and temperature on the ordinate.

Psychrometric chart

Thermodynamic chart including the dry temperature t on the abscissa and the specific humidity w on the ordinate.

Diffuser

Organ allowing the conversion of the kinetic energy of a fluid into pressure

Economizer

Heat exchanger for heating a fluid in the liquid state

Efficiency

Ratio of the useful energy effect on the purchased energy involved

Efficiency of a heat exchanger

Ratio of the greatest temperature increase within the fluids to the difference between the inlet temperatures of the two fluids

Ejector

Organ allowing the aspiration of a fluid thanks to the enthalpy of a motive fluid

Internal energy

Sum of kinetic energies (comparable to the thermal agitation of particles) and potential energies of microscopic interactions (chemical bonds, nuclear interactions) of the particles constituting a system

Purchased energy

sum of all the energies provided to the cycle coming from outside

Useful energy

Net balance of useful energies of the cycle, that is to say the absolute value of the algebraic sum of the energies produced and consumed within it participating in the useful energy effect

Enthalpy

Generalization to open systems of internal energy for closed systems.

Environment

All that is external to the system under consideration and with which it interacts

State of a system

The concept of a state of a system represents the minimum information necessary to determine its future behavior.

Reference evolution

Evolution corresponding to the functioning of components which would be perfect, for which a well-chosen variable or state function remains constant and to which we know how to associate a simple evolution equation

Excess air

Air in excess of the stoichiometric reaction.

Air factor

Multiplicative factor of the quantity of air in a non-stoichiometric reaction as compared with the stoichiometric one, equal to the inverse of the richness.

Flash

Evaporation of a fluid by isethalpic throttling from the liquid state

Working fluid

Fluid used in a thermal machine and undergoing various evolutions or processes

Path function

Quantity which depends not only on the initial and final states of the system, but also on the way in which evolution takes place.

State function

A state function is a quantity whose value depends only on the state of the system, and not on its history.

Mass fraction

Ratio of the mass of a constituent to the total mass of a mixture

Molar fraction

Ratio of the number of moles of a constituent to the total number of moles in a mixture

Fusion

Process which undergoes a substance from solid to liquid

Ideal gas

Fluid model where it is assumed that the size of the molecules and the interactions between them are negligible. Its internal energy and enthalpy only depend on temperature.

Perfect gas

Ideal gas with constant thermal capacities

Steam generator

Heat exchanger producing vapor from a pressurized liquid

Relative humidity

Ratio of the partial pressure of water vapor to its saturated vapor pressure.

Specific (or absolute) humidity

Ratio of the mass of water contained in a given volume of wet mixture to the mass of dry gas contained in this same given volume.

Thermal irreversibilities

Irreversibilities due to temperature heterogeneity arise from the difference in temperature which must in practice exist between two substances when they exchange heat.

Isenthalpic

Evolution during which the enthalpy remains constant

Isobaric

Evolution during which the pressure remains constant

Isothermal

Evolution during which the temperature remains constant

Isoquality

Evolution during which the vapor quality remains constant

Isenthalpic throttling

Adiabatic process without work.

Saturating pressure law

Relation which gives the saturation pressure of a fluid as a function of temperature

Oxycombustion

Combustion carried out with pure oxygen or a mixture of oxygen O₂ and carbon dioxide CO ₂ as oxydizer

Exchanger pinch

Minimum temperature difference between the two fluids which pass through a heat exchanger.

Critical point

State where the pure vapor phase has the same properties as the pure liquid phase. It is characterized by the critical pressure and the critical temperature

Heating value

Enthalpy released by a combustion reaction.

Partial pressure

Pressure that a constituent would exert if it occupied alone the volume V of a mixture, its temperature being equal to that of the mixture.

Pressurizer

Component of a Pressurized Water nuclear Reactor (PWR) used to maintain the pressure in the primary circuit

Extraction

Fraction of the main flow of fluid which is extracted in order to ensure a preheating of the fluid before entering the economizer

Efficiency

Ratio of the useful energy effect on the purchased energy provided

Carnot efficiency

Efficiency of an ideal thermal machine describing a cycle between two heat sources

Isentropic efficiency

Used to characterize the performance of compressors and turbines that are not perfect, so that compression and expansion follow non-reversible adiabatic processes

Polytropic efficiency

Infinitesimal isentropic efficiency

Richness

Ratio of the number of moles of fuel contained in a given quantity of mixture, to the number of moles of fuel contained in the stoichiometric mixture, equal to the inverse of the air factor.

Reheat

Refers to a cycle in which the partially expanded fluid is warmed up before further expansion

Regeneration

Use of some of the heat available after expansion of a fluid to preheat it

Sublimation

Process which a substance undergoes from the solid state to the gaseous state

Supercritical

State of a fluid whose pressure is greater than its critical pressure

Superheater

Heat exchanger for heating a vapor to a temperature higher than that of vaporization

Open and closed systems

A thermodynamic system designates a quantity of material isolable from its environment by a fictitious or real border. This system is said to be closed if it does not exchange matter with the surroundings across its borders; otherwise it is sayd open.

Separator

Organ used to separate a fluid in liquid-vapor equilibrium by dividing its flow into two parts, one corresponding to the liquid, and the other to the vapor

Temperature

Measurement of the degree of agitation of the molecules of a fluid: the more they are agitated, the higher its temperature

Quenching temperature

Temperature at which unburned gases were frozen during certain combustion reactions, and therefore the one that must be used in the law of mass action to calculate Kp and find the composition of the burnt gases.

Thermocoupler

Mechanism that complements heat exchangers by allowing components other than exchange processes to connect to one or more exchange processes to represent thermal couplings

Quality of a two-phase mixture of a pure substance

Ratio of vapor mass to total mass (vapor + liquid)

Irreversible process

A process is said to be irreversible in the following two cases:

the reverse process cannot be carried out without profound modification of the equipment (mixing, combustion, etc.);
it contains a cause of irreversibility of the friction or viscosity type.

Reversible process

We call reversible process between two states of equilibrium 1 and 2 a fictitious evolution which has the following two properties:

it is sufficiently slow in all respects (velocities, exchanges of heat and matter…) so that we can assimilate it to a continuous series of equilibrium states;
it constitutes the common limit of two families of real processes, one of which leads from 1 to 2, and the other from 2 to 1.

Useful work

Work produced by the moving walls of a machine operating in an open system

Aeroderivative gas turbine

gas turbines derived from aviation, variant of a turbojet engine

Nozzle

Organ allowing the conversion into kinetic energy of the enthalpy of a fluid

Vaporization

Process which a substance undergoes from the liquid state to the gaseous state

Vaporizer

Heat exchanger for vaporizing a fluid

State variables

Set of physical quantities (or thermodynamic properties) (temperatures, pressures, etc.) necessary and sufficient to completely characterize a system at a given instant.