Energy storage pt2

why is it bad to store hot air

pressure will decrease as heat is dissipated

what is the most expensive part of compressed energy storage?

container to store compressed air

Isochoric high pressure air storage

constant volume

cheaper

first kg of air better than second etc

Isobaric high pressure air storage

constant pressure

better than isochoric

every kg of air delivers same output work

What is the best geometry for air storage

Sphere better than cylindrical as stress is distributed more uniformly

How can high pressure air be stored

salt caverns

aquifers

pre-existing caverns (mines, tunnels)

Limitation of compressed air

not an effective way to transmit power over large distances due to pressure drop

CAES Configurations

Multi-stage compression with inter-cooling

Multi-stage expansion with inter-heating

Thermal store: none / 1-level / multi-level / thermocline/ 2-tank.

Recuperation from exhaust air

Throttling output air down to fixed turbine inlet pressure

Separate shafts motor+compression and expansion+generation (or the same shaft).

Modular turbine and expansion units (1 / 2 / 3) in parallel.

Drawback of isothermal CAES

heat exchange is relatively slow

what is anadvantage of storing heat over pressure

cheaper

Methods of storing heat

sensible heat

latent heat

thermochemical

Sensible heat storage

heating a material without causing phase change

latent heat storage

heating a material until it experiences phase change

during phase change large amount of energy absorbed

thermochemical storage

uses reversible endothermic / exothermic chemical reactions

A chemical compound is heated up to it’s reaction temperature. A large amount of heat is absorbed (endothermic) to break the chemical bonds (heat of reaction =Δr )

The resultant compounds are stored separately at ambient temperature. When energy is required, the two products are mixed together. The reaction releases the stored heat (exothermic reaction)

Why do we store heat?

Direct use (T close to ambient)

Electricity generation (T > 400 degC)

What are heat pumps used for?

to upgrade low grade heat to a higher temperature

How do heat pumps work?

they move heat from a cold place to a warm place using a refrigeration cycle. A fluid (refrigerant) absorbs heat as it evaporates at low pressure, a compressor raises its temperature, and it releases that heat when it condenses at high pressure. This lets the system deliver more heat energy than the electrical energy it consumes.

The Rankine Cycle

a thermodynamic cycle that converts heat into mechanical energy

Adiabatic Compression: The pressure of the liquid is raised by the pump

Isobaric Heat Addition: The liquid is heated up at constant pressure. The heat source can be a boiler, a thermal energy store,  a heat exchanger, etc. The liquid boils and becomes a vapour

Adiabatic Expansion: The vapour is expanded in a turbine. The expansion of the vapour generates mechanical work. The vapour temperature and pressure drop

Isobaric Heat Rejection: The vapour at a lower temperature is passed through a condenser where a secondary fluid cools it down. It becomes liquid again

Brayton/Joule Cycle

can be open or closed loop, represents gas turbine engine

Adiabatic Compression: The pressure of the gas is raised by the compressor. 

Isobaric Heat Addition: This can be done indirectly via a heat exchanger in a closed loop cycle or  directly as constant-pressure fuel-combustion in an open loop cycle

Adiabatic Expansion: The gas is expanded in a turbine, which produces mechanical work. The temperature and pressure of the gas drop. Most of the work is used for jet propulsion or electricity generation while a small amount is used to drive the compressor

Isobaric Heat Rejection: Cool the air at constant pressure back to its initial condition

Back work ratio

work consumed by compressor / work produced by turbine

Net work for rankine/brayton cycles

turbine work - compressor work

work conversion

work consumed in charge/discharge / exergy out

higher = more exergy loss and more CAPEX

recuperator

type of heat exchanger that improves efficiency by recycling heat from hot exhaust gases to pre heat fluid

How can exergy be destroyed

By irreversible processes:

transfer heat across a temperature difference

throttle a fluid

friction/damping

mixing two different temperature/concentration of fluids

electric resistance heating

throttle

let a fluid pass from high pressure to low without taking work out

what happens when energy is transformed

some exergy is lost. should transform energy minimum amount of times

First law of thermodynamics

energy cannot be created or destroyed, only changed from one form to another

how does heat transfer across a temperature difference destroy exergy

generates entropy which destroys exergy (even though energy loss is minimal)

worse at bigger temperature differences due to irreversibility

how can exergy loss be minimised

increase heat exchanger surface area - samller dT required

counter current heat exchangers

multi-stage heat transfer

cascade heat use - match T to application

how is thermal mass used to minimise exergy loss

thermal mass - mcp, determines how quickly a material’s temperature changes as it absorbs/releases heat. the two fluids must have well matched thermal mass curves (parallel) for efficient heat transfer.

packed bed heat stores

collections of particles - rocks, steel balls, ceramic

behave like contraflow HEX

have hot end and cold end and thermal energy is transfered through layers

Thermocline

moving temperature front that separates the hot region from the cold region as the bed charges or discharges. A gradual change = low efficiency

how can thermal conductivity be improved?

filler

extended surfaces

smaller particles to minimise diffusion path

liquid format thermal energy storage

heat transfer fluid is also the storage medium

can be done in single tank or betewen hot and cold tanks

Limits of liquid format thermal energy storage

freezing/evaporation temperatures

corrosion

thermal stratification

liquid separates into layers of different temperatures

hot = less dense, cold = denser

what is the key to single tank energy storage?

mixing must be minimised

important characteristics of phase change materials (pcm)

melting point in application range

high latent heat of fusion

high thermal conductivity

thermal stability

small change in volume during phase change

non-toxic/non-flammable

cheap

available heat to transfer using PCMs

dT = Theat transfer fluid - Tmelting

loss in exergy increases at lower Tm

Cascade systems

enhance efficiency by layering multiple storage units with decreasing temperature ranges

chemical thermal energy storage

uses reversible chemical reactions and stores energy by breaking and forming chemical bonds

Characteristics of chemical reactions used for storage

must be in application T range

high enthalpy of reaction

reversible

fast kinetics

non-toxic/non-flammable cheap

two tanks system to accomodate phase liberated during charging

limits of chemical thermal energy storage

incomplete reactions (diffusion limitation) causes problems.

solid format thermal energy storage

limited as heat transfer via conduction is poor

perfectly conductive material

temperature inside material is uniform at any moment

How does carnot efficiency vary as heat is extracted?

it drops because as heat is extracted T drops and becomes closer to Tref.

what’s the difference between Brayton-Joule cycle and Rankine cycle

Brayton-Joule cycle working fluid is always a gas

Why is isobaric storage better than isochoric?

compressors and expanders always work with the same pressure of air in the store

all of the air which is contained in the store when it is full can be extracted from the store

the pressure of the air is the same at all states of fill, the average energy stored per kg of air withdrawn remains higher

air in store does not change in temperature as air is withdrawn.