ISAT 413 - Module V: Industrial Systems Topic 1: Cogeneration / Combined Heat and Power (CHP) Basic Concepts of CHP The Benefits of CHP Problems Associated with CHP The Balance of Energy Demand Types of Prime Movers Steam Turbines Gas Turbines

Basic Concepts of CHP Schemes combine electric power generation with heat space-heating and/or specific industrial processes are frequently referred to as Combined Heat and Power (CHP) or cogeneration. Schemes in which thermal energy is produced centrally and distributed via networks for the heating of houses and public buildings are referred to as district heating. For a typical steam turbine power station if the heat rejected in the condenser were utilized the overall efficiency would be increased to about 75% to 80%. Steam can be bled off the steam turbine at a suitable pressure at night when electricity demand is low and used to heat water in a thermal store. Low temperature of rejected heat and remote location of recipients are two reasons to hinder the popularity of CHP.

An Ideal Cogeneration Plant Cogeneration is the production of more than one useful form of energy (such as process heat and electric power) from the same energy source.

Utilization Factor, H (an Example) Steam enters the turbine of a cogeneration plant at 7 MPa and 500oC. One-fourth of the steam is extracted from the turbine at 600 kPa pressure for process heating. The remaining steam continue to expand to 10 kPa. (continue on next page)

Utilization Factor, H (continued) The extracted steam is then condensed and mixed with feedwater at constant pressure and the mixture is pumped to the boiler pressure of 7 MPa. The mass flow rate of steam through the boiler is 30 kg/s. Disregarding any pressure drops and heat losses in the piping, and assuming the turbine and the pump to be isentropic, determine the net power produced and the utilization factor of the plant.

The Benefits of CHP It can use of about two-thirds of the waste heat. Improved national energy efficiency and the preservation of non-renewable energy reserves. Local generation of electricity. Based on the comparison of transmission of the same quantity of energy over the same distance: Cost index = 100 units for electricity; 27 for oil in an underground pipeline; 35 for natural gas; and 67 for hot water using pipeline. Reduction in environment pollution. The more efficient use of exhaust gasses in, say a waste boiler, (where, if supplementary firing takes place, the fuels are more completely burned), will reduce pollution. Investment in industry. More jobs will be created by government investment in city-based CHP schemes.

Another Example of CHP A hotel has an electrical load of 48 kW and a demand for hot water which generates a heating load of 90 kW during a year in which the average load factor is 75%. This demand is currently met from mains electricity and an oil-fired boiler. It is proposed to install a micro-CHP unit which uses natural gas. The installed cost of the units is $24000. Using the data below calculate the payback period. Data: Present scheme: Electricity, average unit charge, 4.1 ¢/kWh. Boiler: efficiency, 70%; fuel cost, 11 ¢/liter; GCV (Gross Calorific Value) of fuel, 40 MJ/liter; annual maintenance costs, $500. Micro-CHP scheme: Engine: gas input, 153 kW; heat output, 90 kW; power output, 48 kW; fuel cost, 32 ¢/therm; annual maintenance costs, $1500. (Note: 1 therm is equivalent to 29.307 kWh)

Another Example of CHP (continued)

Another Example of CHP (continued)

Problems Associated with CHP The financial appraisal of such a CHP project involves making a number of assumptions on future energy demand, fuel prices and availability, taxes, discount rates, maintenance costs etc. Any changes in the manufacturing process may result much higher than the initial investment. Back-up system as spare won’t be generating sufficient savings to offset the initial capital costs. Extra costs may be significant unless the heat user is relatively near the prime mover. Noise and pollution may involves extra expenditure at a time when regulations are becoming increasingly stringent.

The Balance of Energy Demand

The Balance of Energy Demand Energy savings variation with heat/power ratio Graphs: Left: from Eastop & Croft Right: from EES

Types of Prime Movers (1a): Steam Turbines This example involves a back-pressure turbine, which works with an exhaust pressure appropriate to the process heat requirement. Steam leaves the turbine is used for heating purposes and normally then passed back to the boiler as condensate. A back-pressure steam turbine unit is installed by a company to supply heat and power. The steam conditions at entry to the turbine are 40 bar and 500oC. Taking the isentropic efficiency as 0.86, (i) calculate the heat/power ratio of the unit when the turbine exhaust pressure is 3 bar. Assume that the steam is returned to the boiler feed pump as a saturated liquid at 1 bar. (ii) estimate the turbine exhaust pressure which will result in a heat/power ratio of 6:1. Assume that the steam is returned to the boiler feed pump as a saturated liquid at a pressure 2 bar below the turbine exhaust.

Types of Prime Movers (1a): Steam Turbines Plant Schematic for CHP based on back-pressure turbine T-s Diagram for the CHP scheme

Types of Prime Movers (1a): Steam Turbines {Part i: calculate the heat/power ratio} T_1a=500 P_1a=40 h_1a=ENTHALPY(Steam,T=T_1a,P=P_1a) s_1a=ENTROPY(Steam,T=T_1a,P=P_1a) P_2a=3 s_2sa=s_1a h_2sa=ENTHALPY(Steam,s=s_2sa,P=P_2a) eta_isena=0.86 eta_isena=(h_1a-h_2a)/(h_1a-h_2sa) P_3a=1 h_3a=ENTHALPY(Steam,P=P_3a,x=0) ratio_hPa=(h_2a-h_3a)/(h_1a-h_2a)  ratio_hPa = 4.18 : 1

Types of Prime Movers (1a): Steam Turbines {Part ii: calculate the exhaust pressure, P_2 for h/p=6:1} T_1=500 P_1=40 h_1=ENTHALPY(Steam,T=T_1,P=P_1) s_1=ENTROPY(Steam,T=T_1,P=P_1) s_2s=s_1 eta_isen=0.86 h_2s=ENTHALPY(Steam,s=s_2s,P=P_2) eta_isen=(h_1-h_2)/(h_1-h_2s) P_3=P_2-2 h_3=ENTHALPY(Steam,P=P_3,x=0) ratio_hP=6 ratio_hP=(h_2-h_3)/(h_1-h_2) P_2 = 8.28 bar

Types of Prime Movers (1b): Steam Turbines This example involves a pass-out turbine, in which steam is bled from the turbine at some point or points between inlet and exhaust. The remainder of the steam is expanded in the turbine and then condensed in the usual way. The process steam is used for heating purposes and normally then passed back to the boiler as condensate.

Types of Prime Movers (1b): Steam Turbines A pass-out steam turbine unit is installed by a company to supply heat and power. The steam conditions at entry to the turbine are 30 bar and 400oC, and the steam expands with an isentropic efficiency of 0.84 to an exhaust pressure of 0.08 bar. The power output from the turbine is 5 MW. Construct a table which shows how the percentage of steam flow extracted for process heating alters the heat/power ratio based on the steam being extracted at a pressure of 10 bar. Assume that the steam is returned to the boiler feed pump as a saturated liquid at 2 bar, and that the condition line on the h-s diagram is linear. Assume also that the isentropic efficiency remains the same regardless of the steam flow extracted.

Types of Prime Movers (1b): Steam Turbines "Eastop Example 8.2" T_1=400 P_1=30 h_1=ENTHALPY(Steam,T=T_1,P=P_1) s_1=ENTROPY(Steam,T=T_1,P=P_1) s_2s=s_1 P_2=0.08 h_2s=ENTHALPY(Steam,s=s_2s,P=P_2) eta_isen=0.84 eta_isen=(h_1-h_2)/(h_1-h_2s) P_3=2 h_3=ENTHALPY(Steam,P=P_3,x=0) P_A=10 {Assume linear condition line of h-s between points 1 and 2} h_2=ENTHALPY(Steam,s=s_2,P=P_2) h_A=ENTHALPY(Steam,s=s_A,P=P_A) (h_A-h_2)/(h_1-h_2)=(s_A-s_2)/(s_1-s_2)

Types of Prime Movers (1b): Steam Turbines {Assume m_dot_s = the mass flow rate of the steam into the turbine; and m_dot_p = the mass flow rate of steam extracted for process heating} {Q_dot_heat=m_dot_p*(h_A-h_3) {rate of heat delivery for process heating} Q_dot_power=m_dot_s*(h_1-h_A)+(m_dot_s-m_dot_p)*(h_A-h_2) {delivered power}F=m_dot_p/m_dot_s {percentage of steam for process heating}} {F=0} ratio_hp=F*(h_A-h_3)/((h_1-h_A)+(1-F)*(h_A-h_2)) {Solve Table:} F ratio_hp 0 0.00 0.25 0.86 0.5 2.25 0.75 4.85 1 11.46

Types of Prime Movers (2): Gas Turbines A manufacturing company is supplied with 80000 MWh of electrical energy by the area board each year. The company generates its own heat from a gas-fired boiler. It is proposed to replace this arrangement with a CHP scheme based on a gas turbine. A heat exchanger extracts energy from the turbine exhaust to preheat process air; this energy replaces that formerly supplied by the boiler. Using the data below, confirm that the operating heat/power ratio of the CHP system is about 3:1, and estimate the annual saving in fuel costs of the co-generation scheme over the previous arrangement. A plant schematic and the T-s diagram for the processes are shown on the next slide.

Types of Prime Movers (2): Gas Turbines Plant Schematic for CHP scheme based on gas turbine T-s Diagram for the CHP scheme

Types of Prime Movers (2): Gas Turbines Data: Gas turbine: inlet temperature, T1 = 300K; maximum temperature, T3 = 1300 K; pressure ratio, 6:1; all isentropic efficiencies 0.78; for all the cycle take  = 1.4 and cp = 1.005 kJ/kg-K. Generator: efficiency, 0.95. Fuels: average electricity tariff, 4.2 ¢/kWh; gas price, 28 ¢/therm (1 therm = 29.307 kWh). Boiler: overall efficiency, 0.80. Heat exchanger: effectiveness, 0.70; inlet temperature of process air, 300 K. Assume that the thermal capacities of the process air and the turbine exhaust gas are the same. Also assume a combustion efficiency of 100%.

Types of Prime Movers (2): Gas Turbines

Types of Prime Movers (2): Gas Turbines

Types of Prime Movers (2): Gas Turbines