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8/3/2019 Hydrogen-based Energy Utility - Eeerulez.blogspot.in

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SEMINAR REPORT

ON

HYDROGEN-BASED ENERGY UTILITY

SYSTEM

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ABSTRACT

This system stores off-peak electricity and produces hydrogen viaa novel natural gas/steam thermo-electrochemical process. Thefollowing economic analysis indicates that the system can provideon-peak electrical power competitive with natural gas-fueledpeaking-turbines and can co-produce hydrogen at costsconsiderably below that of delivered merchant hydrogen.

Competitiveness is achieved due to :

(1) the cost for this system is allocated between two products andrevenue sources, namely hydrogen and electric energy storage;

(2) a very high 89% utilization factor;

(3) reducing high-cost electrical needs with low-cost natural gas and;

(4) low capital investment.


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INTRODUCTION

In this system the first is a hydrogen/bromine regenerative electrochemicalcell that is well suited for energy storage applications such as peakshaving, load management and other emerging distributed utilityapplications.

A regenerative hydrogen/bromine cell facilitates electrical energy storageby consuming electricity in electrolyzing hydrogen bromide into hydrogenand bromine reactants as stored chemical energy. The hydrogen andbromine are later reacted electrochemically in the cell to produce electricalenergy. Hence, the cell is regenerative (reversible), in that it canefficiently operate as an electrolysis cell producing reactants and consumingelectricity or as a fuel cell consuming reactants and producing electricity.

In effect, the cell operates as a battery, exchanging electrical and chemicalenergy, with one difference: the reactants are stored outside of the cell asopposed to a battery where the reactants are inside. Therefore, to increasecapacity (kWh) it is only necessary to add more reactants rather than morebatteries.


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In effect, the cell operates as a battery, exchanging electrical and

chemical energy, with one difference: the reactants are stored

outside of the cell as opposed to a battery where the reactants are

inside. Therefore, to increase capacity (kWh) it is only necessary to

add more reactants rather than more batteries.

The second component is a chemical reactor that thermo chemically

produces hydrogen bromide (HBr) and carbon dioxide (CO2) from

water (H2O), natural gas (CH4) and bromine (Br2). The

regenerative cell and chemical reactor integrated in a system

produces hydrogen and stores electrical energy


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Discussion

In this system, the hydrogen/bromine regenerative cell is used both as a

fuel cell to generate electricity and as an electrolyzer to produce

marketable hydrogen. Due to its reversible operation, it is used in an

energy storage system, storing and dispatching electricity during off-

peak and on-peak periods.

Off-peak periods are those times of the day (typically at night) when the

utility has excess generating capacity and electricity energy is

inexpensive due to oversupply. On-peak energy is the opposite; the

generating capacity is fully used and electricity is expensive. Thus, utilities have a .time-of-day-rate. for industrial customers, which

reflects the cost difference between on-peak and off-peak energy.Generally off-peak periods are much longer than on-peak periods.

This difference between length of periods provides an opportunity to

produce more hydrogen than what is required for its fuel cell (on-peak

discharging) mode providing excess or supplemental hydrogen bromide

electrolyte is available during the electrolysis (off-peak charging) mode.


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The technology is a thermo-electrochemical process whereby

hydrogen bromide electrolyte is produced from natural gas,

water and bromine.

The integration of this chemical reactor with the regenerative cellprovides a dual-functional system designed to serve both as an

electricity load-leveling device and as a hydrogen producer. The block

diagram below illustrates the integrated dual function system.


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BLOCK DIAGRAM OF ENERGY STORAGE/HYDROGEN

PRODUCTION SYSTEM

Chemical reactor

Ac/dc converter Regenerative cell

Hydrogen

Bromine

Co2

HBr

Off peak electricity

On peak electricity

Hydrogen

water

Methane


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Technical Background

Electrolysis is currently the most practical method for producing

small and moderate quantities of hydrogen. Electrolysis alsoproduces the purest hydrogen. However, hydrogen produced fromwater electrolysis is relatively expensive. This is due to the highcapital cost of the electrolysis cells and electrical energyrequirements. Electricity is an expensive, high-value source ofenergy when compared to other sources such as coal, naturalgas or gasoline. Hence, a process that reduces the amount ofelectrical power required to produce a given amount of hydrogen isvery advantageous.

The theoretical cell voltage of HBr is 0.555 Volts and that of H2O is1.229 Volts. Actual voltage for commercial electrolysis (whichincludes inefficiencies and parasitic losses) generally range from

0.9 - 1.0 Volts and 1.8-2.2 Volts for HBr and H2O, respectively. OneVolt is the equivalent of 26.58 kWh/kg of hydrogen. Thus, theelectrolysis of HBr requires from 21.27-23.91 kWh/kg of hydrogenproduced. Water, in comparison, requires 47.86-58.48 kWh/kg.Accordingly, electrolyzing HBr instead of water can reduce electricalenergy requirements by over 50 percent.


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Additionally, when recombining hydrogen and bromine in a fuel cell, up to 90%

of the chemical energy stored in the reactants can be recaptured as electricity.

This leads to an electric-to-electric efficiency approaching 80% for the system

versus 40% for hydrogen/air and 35% for most fossil fuel-fired powergenerators. In fact,the electrical characteristics of the hydrogen/bromine fuel cell

can be compared to a battery in terms of response time, with the major

difference that it is only necessary to increase the amount of chemical reactant

to increase capacity (kWh). Other benefits afforded by Hydrogen Bromine are

reduced capital and O&M costs .


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Technical Concept

The photochemical reaction between bromine, steam and solar energy at

elevated temperatures for the production of HBr:

H2O + Br2 + solar energy gives rise to2 HBr + O2 .Ho 298 = +138.34

kJ/mol (1)

The HBr is subsequently electrolyzed into H2 and Br2, in a regenerativecell:

2 HBr + Electricity gives rise to H2 + Br2 (2)

The hydrogen is stored for sale or power generation, while bromine is either

stored or returned to the reactor for further HBr production.

In an effort to reduce the capital cost and siting limitations inherent in

using solar energy, it has an alternative reaction for producing HBr. Thisreaction adds methane to equation 1:

2 H2O + 4 Br2 + CH4 gives rise to. 8 HBr + CO2 .Ho 298 = -

256.99 kJ/mol (3)


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Thus the chemical energy available from methane can substitute for solar energy.

This also has the effect of simplifying the process and making it economically

attractive. Preliminary results showed that HBr could be produced frombromine, steam and methane at a conversion rate of 95% at 750C without a

catalyst.

An alternate reaction to produce HBr without CO2, is to react bromine with

methane to produce HBr and solid carbon. The carbon can be used as amarketable product. This reaction as shown in equation 4

2 Br2 + CH4 gives rise to 4 HBr + C .Ho298 = -140.16 kJ/mol (4).

The new hydrogen production processes utilizing the bromine-methane

reactions with regenerative HBr cells is incorporated in its energy storageapproach.


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Hydrogen/Bromine EnergyStorage

ADVANTAGES:

The hydrogen/bromine energy storage system has definite advantages overother battery systems:

The hydrogen and bromine electrodes are fully reversible allowing veryhigh electric-to-electric efficiencies;

The same electrodes can be used as electro catalysts for both chemicaland electricity generation and therefore, the same cell can be used for bothfunctions;

The cell is capable of operating at a high current and high powerdensity in both modes, resulting in lower capital costs;

The reactants for chemical and electricity generation are storedseparately from the cell which makes it cost effective for both peaking andload leveling (weekly cycle) and low-cost capacity (kWh) increases;

Also, the components are low cost (plastics and composites).


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DISADVANTAGES:

The major disadvantages of the hydrogen/bromine energy storage system is

its use of bromine. Bromine, the only liquid non-metal element known, is a

dark red, fuming liquid.It is reactive and corrosive and has a substantialvapor pressure at room temperatures. However, bromination is an essential

chemical process with related industrial safety, material and operating

standards well known. Because of bromines toxicity the proposed system

draws on the vast experience available from the related chlro-alkali industry.


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Design safety features of the hydrogen/bromine energy storage

system include:

1) Selection of highly reliable equipment, only tried and tested equipment;

2)High safety factor, 50% (rather the usual 10%), minimal use of valves and

flanges;

3) All purges and vents discharge through neutralizing and disposal systems;

4) Complete modularization of the system with computer monitoring of alloperations;

5) Enclosure with slight negative pressure with air discharge through neutralizing

wash;

6)Incorporation of monitors to detect incipient failure and interlocking systems tostand-down faulty module while the remainder of the plant operates;

7)Finally, in the event of an undetected chemical leak, passive neutralization

systems are employed. This is achieved by using crushed limestone

beds to neutralize bromine and hydrobromic acid into benign salts.


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Conclusions

The hydrogen produced in this system is used as chemical feedback or as

an energy carrier.

With this system the R&D has been successful on 10 kw program

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