Risto Lahdelma Professor, Energy technology for communities Tel:

Transcription

1 Energiatekniikan laitos District heating engineering - DH Production Risto Lahdelma Professor, Energy technology for communities risto.lahdelma@aalto.fi Tel: Haichao Wang Doctor of HV&AC PhD candidate of Energy Engineering Department of energy technology, School of Engineering,Aalto University Otakaari 4, ESPOO, Finland R.Lahdelma and H.C. Wang 1 District Heat Demand District heat demand depends on the season The variable demand makes it beneficial to apply different technologies to produce base load, middle load, and peak load In addition sufficient spare capacity should be maintained Heat load duration curve R.Lahdelma and H.C. Wang 2 1

2 Heat load curves Q' Qload,b Heat load Peak heating period Peak shaving heat production Heat load Peak heating period Q' Peak shaving heat production Qload,b Basic district heat production Basic district heat production 0 t1 Operation time of basic heat producer t2 d n Time 0 Operation time of basic heat producer n Duration time (a) heat load curve in chronological sequence (b) heat load duration curve Heat load for domestic hot water is not included. R.Lahdelma and H.C. Wang 3 District Heat Demand Base load Continuous use Low operating costs ( /MWh) High usability Technologies: CHP-plants and solid fuel boilers Medium load Continuous use Can run also on partial load Fairly low operating and investment costs ( /MWh, /MW) Technologies: solid fuel boilers, natural gas boilers Peak load and spare capacity Low investment costs ( /MW) Easy and fast start-up Technologies: Oil boilers, natural gas boilers R.Lahdelma and H.C. Wang 4 2

3 Optimal capacity division between base and peak load We have a load duration curve expressed as F = f(h) F = heat load h = hours in year when heat load is F The inverse function is h = g(f) = f -1 (F) The annual energy is obtained by integrating over f or g E = h max 0 f(h)dh = F max 0 g(f)df The hourly load is satisfied by base & peak plants The annual investment costs (annuity, /MW) satisfy I base > I peak The operating costs (mostly fuel costs, /MWh) satisfy C base < C peak R.Lahdelma and H.C. Wang 5 Optimal capacity division between base and peak load Determine the capacity of base load F base to minimize total annual costs Min C = I base F base + I peak F peak + C base E base + C peak E peak E base = 0 F base g(f)df E peak = E E base F peak = F max F base Substituting expressions for E base, E peak, F peak gives C = I base F base +I peak (F max F base ) + C base 0 Fbase g(f)df + C peak (E 0 F base g(f)df) Forming derivative dc/df base and setting it = 0 gives optimum dc/df base = I base I peak + (C base C peak )g(f base ) = 0 g(f base ) = (I base I peak )/(C peak C base ) F base = f((i base I peak )/(C peak C base )) R.Lahdelma and H.C. Wang 6 3

4 CHP for District Heat CHP for district heat Back pressure turbines and bleeder turbines can produce heat along with power Bleeder turbines providing heat at different pressure levels can heat up the DH water in multiple phases R.Lahdelma and H.C. Wang 7 DH in Finland (a) Heat production (b) District heat production by fuels Source: Statistics Finland, 2012 R.Lahdelma and H.C. Wang 8 4

5 Fuel use in DH and by production mode The fuel distribution is given below 17% Condensing power production 11% Seperate heat production 72% Combined heat and power production Source: Statistics Finland R.Lahdelma and H.C. Wang 9 Heating value of fuels The heating value of fuels means how much heat energy per kg is obtained (MJ/kg) from solid or liquid fuel per normal cubic meter (MJ/Nm 3 ) for gas fuel The higher/upper heating value (HHV) of fuels is determined by bringing all the products of combustion back to the pre-combustion temperature (in particular, condensing any produced vapor) The lower heating value (LHV) is determined by subtracting the heat of vaporization of the water vapor from the higher heating value The energy required to vaporize the water is therefore not realized as heat. The gross heating value accounts for water in the exhaust leaving as vapor, and includes liquid water in the fuel prior to combustion This value is important for fuels like biomass or coal, which will usually contain some amount of water prior to burning The actual concentration of water may vary much depending e.g. on storage conditions For peat, quality classes have been defined according to water concentration P10: 30-50% P12: 30-45% P13: 20-38% R.Lahdelma and H.C. Wang 10 5

6 Heating value of fuels Fuel Higher heating value Lower heating value Gross heating value (typical) Coal Heavy fuel oil Light fuel oil Wood Bark - Pine Spruce Birch Black liquor - Pine Birch Peat Natural gas (MJ/kg) Natural gas (MJ/Nm 3 ) R.Lahdelma and H.C. Wang 11 Combined heat and power production (CHP) The goal is to maximize the amount of useful heat and/or power energy produced from fuel Simultaneous production of heat and power yield much higher over efficiency compared to separate production of power and heat In particular, the condensing power plant converts only about 35% of the fuel into power Consumption ratio q = F fuel /(P+F DH ) P = electric (net) power F DH = district heating power F fuel = fuel power Efficiency of process h = (P+ F DH )/F fuel = 1/q R.Lahdelma and H.C. Wang 12 6

7 Efficiency in condensing power production Fuel power Boiler efficiency h b Power of hot water or steam production Efficiency of transmission of steam to turbine h t Input steam power of turbine Electro-mechanical efficiency of turbine and generator h mg Efficiency due to condense loss h cond Effiency of turbo-generator h tg = h mg h cond Gross electric power Power plant efficiency (subtracts plant power consumption) h pp Transformator efficiency h tr Net electric power F fuel h b F fuel h b h t F fuel h b h t h tg F fuel h b h t h tg h pp h tr F fuel R.Lahdelma and H.C. Wang 13 Consumption ratio of condensing power production The consumption ratio of condensing power production is q cond = 1/(h b h t h tg h pp h tr ) = q b /(h t h tg h pp h tr ) where q b = consumption ratio of boiler The consumption ratio of a condensing plant is normally depending on size and technology of plant The consumption ratio of a bleeder turbine is obtained by subtracting the extracted heat q bl = q cond - F DH q b /P The efficiency depends thus on the DH load Fuel power depends on consumption ratios for heat and power F fuel = q cond P + q b F DH R.Lahdelma and H.C. Wang 14 7

8 Heat pump QU TU High temp. Liquid Condenser High temp. Vapor Low temp. Liquid & Vapor mixture Expansion Valve Compressor Evaporator W Elec. EHP Mechanical energy Thermal activated THP Low temp. Vapor QL TL Working fluid Schematic diagram - basic principle of a Heat Pump R.Lahdelma and H.C. Wang 15 Heat pump What is a heat pump? A refrigeration system for heating using the heat energy released from the condenser. Cooling by inverse circulation. Working in the same way as a refrigerator, but with different purposes; and with different working temperature ranges. Heat pump classification, in general, Air source Water source, sea, lake, river, sewage Ground source, underground pipes (horizontal and vertical) R.Lahdelma and H.C. Wang 16 8

9 Heat pump Coefficient of performance (COP) Energy balance of a heat pump QU=W+QL COP for heating: QU QL + W e H = = W W Means the amount of heat produced per power (W) used, 3+ for efficient system; Theoretical maximum TU e H = TU TL TL is vaporization temperature [K]; TU condensation temperature [K]. COP for cooling e C = e H -1 R.Lahdelma and H.C. Wang 17 Advantageous Heat pump Characteristics enables use of low temperature heat COP approximately 3 improves fuel-efficiency of direct electric heating Disadvantageous High investment costs Maintenance Complex machinery COP decreases when load increases Low condenser temperature R.Lahdelma and H.C. Wang 18 9

10 Trigeneration Four basic components, 1. prime mover, PM; 2. electricity generator, G; 3. thermal recovery system (exhaust gas and engine cooling); 4. cooling energy production system, which is usually adopted as a THP R.Lahdelma and H.C. Wang 19 Trigeneration A. R.Lahdelma and H.C. Wang 20 10

11 Trigeneration B. R.Lahdelma and H.C. Wang 21 Trigeneration C. R.Lahdelma and H.C. Wang 22 11