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Thermal Analysis And Part Load Engineering Essay

This paper briefs the CHP technology, its variants and it advantages over the conventional systems. Major part of the paper explains about the gas turbine CHP system and its technology advancements for applications at a smaller scale levels. It includes perceptive nature of energy demands of commercial/domestic application and feasibility study of CHP installation for such purposes. Design and performance analysis of a gas turbine CHP unit for a considered Commercial building application is done and also performance of unit at part load variations, depicting the demand changes throughout a year is studied. Cost analysis of the unit and its comparison to annual costs of energy consumptions for conventional systems is also performed.

Growing need for power and energy requirement, limited resources at hand have kept human society in constant search for efficient technologies in energy sector. Conventional systems ‘power only’ or ‘thermal only’ have less efficiency with energy lost like the thermal waste and fuel losses. The heat energy, left to atmosphere during power generation can be used for other utilities or heating requirements. Better utilization of power, better utilization of fuel, increased overall efficiency, reduced energy costs and low emissions can be made possible through combined generation of heat and power.

Combined heat and power (CHP), a distributed generation system, is an electric power generating unit placed at or near customer facilities to supply onsite multiple energy needs. Unlike central power stations which transmit power through grids, CHP units are decentralized systems placed onsite as it is inefficient to transmit heat energy to long distances. Though CHP (also known as Cogeneration) came to existence way long back, factors like economic feasibility, technology and government laws kept its full potential out of reach to masses, until recent advancements. Presence of various CHP technologies, made it possible for its installation over wide range of power and heat requirement. All CHP systems consist of basic individual components of heat engine, generator and heat recovery unit.

Figure 1 shows the comparison between convention system and CHP unit with units of fuel used in both the systems along with their individual efficiencies

Figure 1

The type of CHP system depends on the type of heat engine (prime mover) it is driven. These include reciprocating engines, combustion or gas turbines, steam turbines, micro turbines, and fuel cells. Various fuels can be used as source of energy in different CHP systems. Although mechanical energy developed from the prime mover is most often used to drive a generator to produce electricity, it can also be used to drive rotating equipment such as compressors, pumps, and fans. Thermal energy from the system can be used in direct process applications or indirectly to produce steam, hot water, hot air for drying. Onsite CHP installation can be connected to central grid, and power can be reverse supplied during no load conditions. In remote areas CHP units are off- grid and much preferred over central power supply. It can also be used along side of conventional system and loads are shared between both of them.

The selection of appropriate CHP system for a particular purpose depends upon factors like energy requirement, economic feasibility, heat to power ratio, available fuels etc. Most of the manufacturers provide an initial feasibility and cost analysis to clients for installation of a CHP unit and the selection of its kind.

Table 1

Summary of CHP Technologies

CHP system

Advantages

Disadvantages

Available sizes

Gas turbine

High reliability.

Low emissions.

High grade heat available.

No cooling required.

Require high pressure gas or in-house gas compressor.

Poor efficiency at low loading.

Output falls as ambient temperature rises.

500 kW to

250 MW

Micro turbine

Small number of moving parts.

Compact size and light weight.

Low emissions.

No cooling required.

High costs.

Relatively low mechanical efficiency.

Limited to lower temperature cogeneration applications.

30 kW to 250 kW

Reciprocating

Engines

High power efficiency with part-load operational flexibility.

Fast start-up.

Relatively low investment cost.

Can be used in island mode and have good load following capability.

Can be overhauled on site with normal operators.

Operate on low-pressure gas.

High maintenance costs.

Limited to lower temperature cogeneration applications.

Relatively high air emissions.

Must be cooled even if recovered heat is not used.

High levels of low frequency noise

Low speed

(102-514 RPM)

4 - 75MW

High speed

(1200RPM)

Less than 4MW

Steam turbines

High overall efficiency.

Any type of fuel may be used.

Ability to meet more than one site heat grade requirement.

Long working life and high reliability.

Power to heat ratio can be varied.

Slow start up.

Low power to heat ratio.

50 kW to 250 MW

Fuel cells

Low emissions and low noise.

High efficiency over load range.

Modular design.

High costs.

Low durability and power density.

Fuels requiring processing unless pure hydrogen is used

5 kW to 2 MW

The above table gives a brief knowledge about different CHP technologies that are most widely used today.

Power to heat ratio is one of the most vital technical parameters influencing the selection of cogeneration system. The power-to-heat ratio indicates the proportion of power (electrical or mechanical energy) to thermal energy produced in the CHP system. For any application, power to heat ratio is considered and compared with the chp technologies during feasibility analysis

GAS TURBINE CHP SYSTEM

Gas turbines are one of the cleanest means of generating electricity with its low emissions And it produce high-quality exhaust heat that can be used in CHP configurations to reach overall system efficiencies of 70 to 80 percent. General gas turbine units range is from 500KW to 250MW, over the time smaller size gas turbines are being manufactured with promising efficiencies which opened up huge market for domestic and commercial CHP installations. Gas turbines with range of 30KW to 200 KW called microturbines are responsible for success of CHP technology within general population. Recent advancements lead to further research into this technology, developing much smaller units (ultra microturbines) used for purposes down to individual needs like powering a PC or laptop etc, though this technology is yet to deal with lots of technical and economical issues. Gas turbine CHP units run on a wide range of fuels, it’s easy operation, low maintenance and energy cost and with comparatively low emissions make it one of the best CHP system in use.

The working of gas turbine is based on the Brayton thermodynamic cycle. The air is compressed in a compressor and it is burnt with fuel in the combustion chamber giving out high speed, high temperature exhaust gases which are expanded in turbine to produce mechanical energy and is converted into power by a high frequency generator. Total power developed is always in excess to the small part of mechanical energy used to run the compressor, which is coupled to turbine by a shaft. The turbine, generator and compressor can be coupled by single axial shaft or two shaft based on the requirement.

A gas turbine CHP unit consists of a gas turbine with its exhaust connected to a heat recovery system for capturing thermal energy. The hot exhaust gasses exiting the turbine can be used for

Direct heating

Generating steam for utility/process

Absorption cooling

Figure 2 shows the basic gas turbine CHP unit with various possible heat recovery systems.

Note: If exhaust heat is further used to develop power by using heat exchanger and steam turbine, it is NOT a CHP unit (The thermal waste must be for end use).

Figure-2

Various enhancements for basic cycle gives additional benefits like Fuel consumption may be decreased by preheating the compressed air with heat from the turbine exhaust using a recuperator or regenerator; the compressor work may be reduced and net power increased by using inter-cooling or pre-cooling.

DEMANDS OF AN APPLICATION AND THE PROPOSED BASIC CHP MODEL

In the present study, design of a gas turbine CHP unit is to be carried out for a commercial building application for which annual heating and power requirements are as mentioned in the table no 2. Though the demand for both heat and power changes in a day, CHP unit and its components are selected based on the peak demand required in a year. Load factor is used during the calculation of annual energy consumptions and cost estimation. Table-3 shows the Annual consumption of both electricity and heating, considering the monthly load factor and 24hr /7days time operation.

Table 2

MONTH

JAN

FEB

MAR

APR

MAY

JUN

JUL

AUG

SEP

OCT

NOV

DEC

Heating peak

Demand (KW)

75

78

70

62

55

42

38

38

40

56

62

65

Peak power

(KW)

40

40

40

40

40

40

40

40

40

40

40

40

Load factor

.6

.6

.5

.5

.5

.5

.5

.5

.5

.5

.6

.6

Table 3

Power

Heating

Annual Consumption

186.720 MWH

268.173MWH

Peak demand in an year

40KW or 136576.97BTU/hr

78KW or 266325.9 BTU/hr

Figure-3

The figure 3 shows the proposed CHP system shown. It’s a simple gas turbine cycle with a heat recovery system and having a single axial shaft turbine unit, which is advantageous and easy to run, compared to double shaft unit.

2.1 PROPOSED MODEL DESIGN FROM THRMAL CYCLE AND ENERGY CALCULATIONS:

Gas turbine works on the Brayton thermal cycle. This consists of isentropic compression and expansion, combustion and exhaust. The selected cycle is an open and a simple gas turbine cycle for the CHP unit, where the gas exhaust is not added to the compression. An actual gas turbine cycle is little different from ideal Brayton cycle. The compression and expansion processes are not isentropic, due to various losses and mechanical constraints. The combustion process does not occur at constant pressure, there will be slight losses in pressure. The T-S diagram for an actual cycle and ideal cycle is shown in the figure-4 below.

Figure 4

Note that for open process, there won’t be the process 4-1 (from figure) i.e. addition of exhaust to intake. At the intake, in actual conditions the compressor pressure P1, is less than the atmospheric pressure due to suction of compressor, P1 < Patm (atmospheric pressure). The pressure ratio of compressor (r) gives the compressor outlet pressure P2.

Actual compressor outlet temperature (T2) is different from isentropic temperature (T21) and it can be calculated as

Where T1 is inlet atmospheric temperature and ‘k’ is the heat capacity ratio of air and

‘c’ is the compressor efficiency

There is pressure drop in the combustion chamber due to the heat loss to walls, which can be expressed through efficiency of chamber (cu). Usually these losses are around 2%. So, the inlet turbine pressure (P3) can be obtained by multiplying it by the efficiency of chamber, this results in change of actual air -fuel ratio (AFRA) to that of stoichiometric air-fuel ratio (AFRS).

The turbine inlet temperature (T3) can be calculated for a desired work output (W) at a particular pressure ratio (r) as

= Fuel specific heat capacity ratio , = Turbine efficiency.

= fuel heating value, = Flue gasses constant pressure specific heat capacity

= Fuel heat capacity at constant pressure , = Air heat capacity at constant pressure

From the actual air-fuel ratio AFRA we can calculate the mass flow rate of exhaust Flue gasses as = (Total mass of gasses i.e. AFRA + 1) * ( mass flow rate of fuel)

The mass flow rate of fuel can be obtained by fixing i.e. selecting a particular value of the air flow rate (MA ) at the intake of compressor and cross multiplying it with AFRA.

The exhaust energy (EX) can be calculated from exhaust flow rate (MFl )as

Note that T4, Actual exhaust gas temperature is comparatively high than the isentropic turbine exhaust temperature (T41), which can be obtained as

= The specific heat capacity ratio of flue/ exhaust gasses

The , value is nothing but , as there are losses in both combustion chamber and turbine, the inlet turbine pressure or combustor exit pressure (P3) will not be the same as compressor outlet pressure P2 and the outlet turbine pressure (P4) slightly high than atmospheric pressure (Patm) due to turbine losses. So, won’t be equal to (r) unless these losses are accommodated.

In order to design the CHP for desired output i.e. peak demand, selection of parameters must be done and these remain constant for entire operation. The table 4 gives selected CHP parameters, standard values, assumptions and outputs calculated from the cycle analysis.

Table 4

Used fuel & its properties

Natural gas

High heating value

Flue gasses properties

Air properties

24000BTU/lbm

17.2 : 1

2.34kJ/kg K

1.27

1.35

1.126kJ/kg K @6000C

1.4

1.005kJ/kg K

Assumptions / selected component parameters

1.Compressor :

air flow rate ( constant )

pressure ratio

efficiency

inlet pressure

2.Turbine:

Efficiency

3. Assumed combustor efficiency

0.3kg/s

2.5,

80%,

0.98atm

85%

98%

Sized CHP system using energy and thermodynamic analysis@ peak load

CHP rated out put

40KW @ 15deg.C Ambient temp.

Heat rate

13135 BTU/KWH

Fuel energy flow rate

(Fuel mass flow rate)

525139BTU/hr

(0.003 kg/s)

Exhaust energy flow rate

Exhaust mass flow rate (exhaust gasses)

442100BTU/hr

0.203 kg/s

Electrical efficiency a

26%

Exhaust gas temp at the turbine outlet

820K(547deg.C)

Compressor Exit Temperature

Simple cycle efficiency

395.75K

27.95%

Assumed 10% losses in generating electricity from shaft output.

The both compressor and turbine components are selected from generally available turbochargers. The flow maps provided by manufacturer are used for the operation of compressor and turbocharger over a wide range of pressure ratios and air flow rates without any surge/stall. Compared to gas turbines used for high power generation, the compressor of small scale power units and turbochargers have pressure ratios ranging from 1.2 to 3 over a range of air flow rate. The combustion chamber is placed between compressor outlet and turbine inlet. 3-phase generator, along with rectifier and inverter is used to generate power.

LOAD ANALYSIS AND DISCUSSION OF RESULTS

The performance of the turbine varies with the variations in any one of the factors like load, ambient temperature, altitude, pressure ratio and ambient pressure. Moreover these variations affect the heat energy available for the heat recovery system thus affecting the CHP overall efficiency. In the present study performance calculations are confined to the part loads and change in ambient temperature and the rest of factors are considered to be constant. The performance variations calculated for part loads and change in ambient temperature within these loads are shown in table 5.

In order to properly evaluate the performance of the system, the analysis is done taking two part load values of power and within each part load the performance is done against two different ambient temperatures.

It is observed that with the increase in ambient temperature for a constant inlet and pressure ratio the electrical efficiency decreases. Though the decrease in efficiency is low the heat rate is increasing at high rate and more exhaust heat energy is obtained with high exhaust temperatures. This change can be observed in hot seasons.

With increase in load the net heat rate gets reduced and the net efficiency increase. Though we can’t estimate the nature of increase in efficiency through present study, but in many CHP manufactured units performance charts shows it to be exponential increase and then remains almost constant from high part loads to the full loads.

Table 5

Load

Ambient temp

T1 0C

C

%

net

%

Fuel energy

Flow

BTU/hr

Heat Rate

BTU/KWH

Exhaust

Energy flow

BTU/hr

Exhaust

Flow rate kg/s

T4

K

(II)

HEAT

DEMAND

BTU/hr

Heat1 recovered

BTU/hr

O

%

FERC

%

16

kw

15

27

24.3

190480

11905

170827

0.21

500

109262

130682

96

63

163892

130682

96

63

40

25.7

23.1

207620

12976

182330

0.214

528

109262

139480

92

60

163892

139480

92

60

30

kw

15

28.6

25

380954

12698

307298

0.198

670

109262

235083

88.9

57.7

163892

235083

88.9

57.7

40

26.3

23.7

398097

13270

331200

0.21

698

109262

253370

88

57.5

163892

253370

88

57.5

Heat recovered is calculated assuming the heat exchanger efficiency of he = 85% and available heat from turbine exhaust is 90%

net is net electrical efficiency%

Cy is thermal cycle efficiency %

T4 is turbine Exhaust temperature in K

Over is overall CHP efficiency %

FERC is overall efficiency of CHP according to Public utilities regulatory Act 1978[1]

= (Net Power + Heat recovered / 2) / heat input

[1] CHP catalogue, EPA-U.S., www.EPA.gov

2.3 SEASONAL HEAT LOAD VARIATIONS AND OPTIMIZATION OF CHP:

The most important observation of all would be that, though the power to heat ratio at an annual rate is around 0.69. The demands for both power and heat are highly different when considered on daily and monthly basis. For example during the months of summer the heating requirements get drastically reduced and power demand increases. During the operation of turbine unit at these conditions more exhaust heat is released, whereas requirement of heat is very low. Thus much of fuel energy is lost in heat to atmosphere.

During the cold seasons the heating demand is huge compared to power. During certain operating conditions exhaust heat may be inefficient to fulfill the heating demand. By this we can say that design of CHP system hugely depends on the geographical location and weather conditions.

These variations can be observed in Table 4. Where in the column (II) has given the heat demands for a particular load and temperatures. These demands are taken at 40% and 60% of peak heating load. The heat recovered from exhaust energy is calculated for each condition assuming that only 90% of exhaust heat available for heat recovery and heat exchanger efficiency is 85%. The shortcomings for the heat load are highlighted in Red and for huge surplus heat recovered are marked in Blue.

General optimization technique would be using a regenerator at the entry of compressor and the heat is provided from the exhaust turbine gasses before letting through the heat recovery system. The advantage of using this component is fuel consumption gets reduced, but net exhaust energy available for heat recovery gets reduced.

Other option would be usage of Thermal energy storage tank, which can store energy and can be delivered through a period of time of when it is required. Various salts of chlorides or fluorides can be used for energy storage at molten stage. These can be used at small scale level with good efficiency. CHP units connected to the local or central grid can sell the surplus power at considerable value. This can increase the annual savings and make the system a profitable venture

COST ANALYSIS:

Annual energy consumptions that are given, considering the monthly load factors and 24hrs in a day.

Power

Heating

Total Annual Consumption

186.720 MWH

268.173MWH

The individual cost rates using conventional systems and the annual cost of heat and power from conventional sources are obtained, shown in the table

Gas

2.817pence/KWH a

Annual cost for heating

7,697£

Electricity

9.59pence/KWH a

Annual cost for power

17,905£

a ,both the prices were obtained from 4th quarter energy prices of 2010 year for non domestic consumers from department of energy and climate change UK, www.decc.gov.uk

Considering the availability of CHP unit to be 8400hrs and the running at full load@40KW for 65% of the time. The annual consumption costs and additional costs are calculated for the CHP unit as shown in below table.

Note that the fuel consumption obtained earlier during performance analysis is 525139BTU/hr. and heat recovered from heat exchanger is 99KW, considering 90% availability of turbine exhaust energy and efficiency of heat exchanger at 85%

Fuel consumption rate@ peak load

525139BTU/hr

Annual fuel consumption=

fuel rate*available hrs*%load factor

2867.26MMBTU or

839.75MWH

Fuel cost rate1

4.96£/MMBTU

Annual CHP fuel cost

14,224£

Annual operation and maintenance

=0.00598£/KWH of power1

1,118£

Thermal credit (Saved)

=0.0287£/KWH of heat recovered1

15,514£ [A]

Total cost to generate power

=0.0369£/KWH of power1

6,890£

Initial capital (CHP cost)

=7,176£/KW of rated power1

287,050£

[A], note that the thermal credit i.e. saved heat amount is huge. This is due to the fact that the annual heat recovered energy is higher than required annual heat demand, thus result a surplus output. So, if the CHP is connected to a thermal energy storage tank, we could sell the energy and gain profit.

1, all the economic analysis is done based on the CHP financial analysis presented by Environmental Protection Agency EPA, USA. www.epa.gov

CONCLUSIONS

CHP installations are possible only when there is considerable heat demand to that of powers for an onsite purpose i.e. the power to heat ratio is an important parameter while deciding the possibility of CHP unit. For power to heat ratio reaching value of one, CHP unit is not recommended.

The designed gas turbine CHP unit for considered commercial purpose is efficient and fulfils the requirement, with shown results in performance analysis. Due to the seasonal changes in demand for both power and heat, it was found that there are situations of shortcomings and also surplus heat recovery. This is also observed to be same for any other CHP units and this can be overcome by installation of thermal energy storage equipment.

Compressor pressure ratios, air flow rate and turbine inlet temperatures are of major importance in deciding the efficient operation of turbine, a system monitor is needed for a turbine in order to run efficiently and safely.

From the economic analysis of the designed CHP unit, it is clear that for almost same fuel input cost to that of conventional system, both heat and power demands can be met with good amount of savings and also be a profitable venture. Even with considering the initial capital, profitable returns can be expected within short span of time.

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