Learning objectives and outcomes

Transcription

1 Ene Energy Systems for Communities Micro-Cogeneration Kari Alanne Senior University Lecturer, D.Sc (Tech.) Learning objectives and outcomes After this lecture the student will know the definitions and technologies understand the key principles of operation and system integration recognize the future challenges and the trends of development related to micro-cogeneration.

2 Lecture outline Background and definitions Technology and market Micro-cogeneration systems Operational challenges Trends of development Distributed energy Sustainable development Reciprocating engines Micro-turbines Fuel cells Integrated systems Efficiencies Thermal and electrical mismatch Power control Load management Energy storages Polygeneration Hybridization Background Networking Decentralization Scarcity of natural resources Sustainable development Efficient use of energy and raw materials Flexibility and scalability Utilization of local resources

3 Definitions General: CHP: Combined Heat and Power electricity and thermal energy in small units close to boiler in a hydronic heating system, which simultaneously produces heat & electrical Technical: EU Directive on microcogeneration: electrical power less than 50 kwe Mini-CHP electrical power > 50 kw e European Committee for Standardization (EN50438): 16 A per phase in three phase (25 A single phase) Domestic scale microcogeneration (DCHP): one unit practically: less than 5 kwe Micro-CHP technologies fuel cells (FC) Operational principle: inverse electrolysis operational temperatures Poly-Electrolyte M (Solid-Oxide Fuel Cells, SOFC) Fuel: hydrogen, reformed natural gas Efficiency: electrical efficiency 40 % overall efficiency % electrical power / heat flow ~ 1.0 (PEM) Market status: emerging technology Estimated installed costs: e kw e plants, full market) Source: Center for Fuel Cell & Hydrogen Research

4 Micro-CHP technologies Stirling engines (SE) Operational principle: reciprocating engine, combustion outside the cylinder Expansion cylinder: high temperature is maintained using external thermal source Fuel: natural or biogas, gasoline, diesel, LPG, various liquid or solid fuels Efficiency: electrical efficiency % overall efficiency % electrical power / heat flow ~ 0.3 Market status: emerging technology Estimated installed costs: plants, full market) e e Compression cylinder: cooled down using e.g. water circulation Alpha type Stirling engine Micro-CHP technologies other Internal Combustion Engine (ICE): conventional reciprocating engine, combustion inside the cylinder natural or biogas, diesel, gasoline electrical efficiency % on the market at 1 kw e < Micro-turbines (MT): conventional gas turbine process natural or biogas, diesel, gasoline, alcohols electrical efficiency % at > 25 kw e on the market at 25 kw e < (not practical for DCHP) Steam engines: based on Rankine cycle (or Organic Rankine cycle, ORC) piston steam engine (PSE) rotary steam engine (RSE) electrical efficiency % at < 10 kw e

5 Market requirements for micro-chp Favorable: good availability of products supporting political definitions strong customer demand and increasing purchasing power utility interests Infavorable: competition against other energy sources high investment costs high fuel costs poor system performance Major manufacturers and products Technology Manufacturer Product ICE Honda Ecowill ICE Senertec Dachs ICE Vaillant Ecopower SE Whisper Tech WhisperGen SE Solo Stirling 161 FC (SOFC) Acumentrics AHEAD FC (PEM) Vaillant FCU 4600 MT Cummins Cummins For self-

6 Images of micro-chp products I Honda Ecowill ICE Whispergen SE Images of micro-chp products - II Acumentrics AHEAD SOFC Turbec T100 MT

7 Domestic micro-chp (DCHP) concept ELECTRICITY IMPORT/EXPORT STORAGE EXHAUST 5-15% Fuel 100% micro CHP unit ELECTRICITY 15-25% Heat 70% lighting appliances building services space heating domestic hot water (DHW) Requirements: Efficient operation Affordable investment and operation costs Good power control Low noise Minimum space requirement Extended life span Low emissions Characterization of micro-chp technologies Control Noise Size Life span Emissions FC +( ) ++ +(+) SE + + +() ICE MT + In the above table, the scale varies from very good (++) to very poor ( ). Controllability and emissions are related to the fuel, emissions also to the efficiency.

8 Comparison of micro-chp technologies - I Internal combustion engines Stirling-engines Costs Electrical Efficiency Lifetime Size Loudness Loudness Costs Electrical Efficiency Lifetime Size Fuel Emission Fuel Emission Power control Power control Comparison of micro-chp technologies - II Fuel cells Micro Turbines Costs Electrical Efficiency Lifetime Size Loudness Loudness Cost Electrical Efficiency Lifetime Size Fuel Emission Fuel Emission Power control Power control

9 Fuel Air Micro-CHP plant Exhaust gas out Auxiliary burner Water out Heat recovery Exhaust gas Water in Pre-handling of fuel and air Energy conversion module Mechanical power or electricity (DC) Power conditioning module Electricity output (AC) Electricity (AC) for ancillaries Electricity to grid Exhaust gases Integration of micro-chp plant into building CHP plant >80ºC Fuel and air Electricity to HVAC, lighting and appliances Cold water Buffer storage ºC* Controller Circulating pump * The storage temperature is controlled using heat sink and auxiliary burner, when needed. Hydronic radiators network or floor heating ºC Domestic hot water 55ºC

10 Electrical efficiency: el Thermal efficiency: th Efficiencies Electricity output (AC) Fuel input to the energy conversion module Heat recovered to water circulation Fuel input to the energy conversion module Annual thermal efficiency: a Annual thermal consumption of space heating and domestic hot water Annual fuel input to the micro-chp plant incl.auxiliary burner Part-load efficiency Part-load efficiency is the efficiency representing operating conditions, when the plant is driven at electrical power different from its specific (nominal) power Overall efficiency is the proportion of utilizable energy of the fuel input to the energy conversion module the sum of electrical and thermal efficiencies Efficiency [%] 90 % 80 % 70 % 60 % 50 % 40 % 30 % 20 % 10 % 0 % Electrical power / Nominal power Electrical Thermal Overall Example: Solid-Oxide Fuel Cell

11 Demand profiles a challenge to micro-cogeneration Standard house, Helsinki Passive house, Helsinki Electricity/heat [W] Electricity/heat [W] Time [h] Time [h] Electricity [W] Heat (standard house) [W] Electricity [W] Heat (standard house) [W] Thermal and electrical mismatch P Electricity purchased from the grid/discharged from the batteries (red lines) Duration curve Example: Micro-CHP plant operates at constant power Electricity to storage/grid (blue lines) Electricity to domestic use (green lines) 8760 h t

12 Example Duration curves for electricity and heat can be assumed linear as beside. P [kw] 2 kw Electricity a) Calculate annual consumption of electrical and thermal energy. 0.5 kw 0 h [kw] 8760 h t [h] b) Calculate annual operating hours of an ON/OFF-controlled heating boiler (specific thermal output 17 kw). 10 kw Heat 0.5 kw 0 h 8760 h t [h] 1. Annual consumptions: Solution area between t-axis and duration curve electricity: W = heat: Q = 2. Annual hours of operation: kwh kwh An ON/OFF controlled boiler has two (2) operational states: 0 kw and 17 kw. The task is to estimate how many hours per year the boiler must operate at 17 kw to generate kwh thermal energy. Q t t Q kwh a h kw a

13 Operational strategies Aim: to find optimal match between electrical and thermal demand and supply Methods: power control load management electrical and thermal storages Power control 1. Electrical load tracking mode, thermal excess is stored or dumped, thermal shortage generated by auxiliary burner and/or discharging the thermal storage 2. Thermal load tracking mode, electrical excess is stored or fed into the grid, electrical shortage satisfied by grid electricity or by discharging the storage 3. Operation at constant power (base load), the employment of thermal and electrical storages, heat sink, auxiliary burner and grid, when needed

14 Temperature control of buffer storage The purpose of the buffer storage: 80 to deliver heat to the hydronic heating system to shave the peak thermal demands Preset threshold values for storage temperatures determine the on/offoperation of the micro-chp plant. The temperature of supply water to the radiator network is controlled by mixing supply and return water according to the outdoor temperature. Supply water temperature [C] Outdoor temperature [C] Power control challenges Only on/off operation available (common for present day micro-cogeneration plants based on ICEs) Long start-up and shutdown periods may be required (SEs) Substantial fuel demand at start-up phase (SEs) Limited dp/dt (SOFCs) Low part-load efficiency (SOFCs) In general: steady demand close to specific power output is preferable in the sense of micro-cogeneration.

15 Example Consider the previous example. Assume the 17 kw boiler is replaced by an ON/OFF-operated micro-cogeneration plant capable of generating 17 kw thermal power and 1.7 kw electrical power. The operation is completely thermal tracking (the number of annual operating hours is the same as calculated previously). Determine the annual shortage, surplus and on-site electricity in the conditions of minimum mismatch (maximum onsite electricity). Solution 1. Annual generated electricity: 1.7 kw 2705 h/a = 4599 kwh/a 2. Demand exceeds production at kw kw P 3. Surplus: kwh kw h 78 a 4. On-site: c t c 0.5 kw 8760 h kw h 8760 h 1752 h ( ) kwh/a = 4521 kwh/a 5. Shortage: ( ) kwh/a = 6429 kwh/a t c P c 1.54 kw 2 kw 1.7 kw P c 0.5 kw 2 kw 1.7 kw P [kw] 0 h P [kw] t c MIN mismatch t = 2705 h MAX mismatch 8760 h t [h] For self-studying: Calculate the example for MAX mismatch. Reflect how the mismatch impacts the economic viability of micro-cogeneration. Or does it? 0 h 8760 h t 8760 h t [h]

16 Load management A procedure to adjust electrical demands rather than the output of the plant Examples: Forced switch- - stoves and ovens Limited simultaneous use of electrical appliances Seasonal (long-term) thermal storages Thermal surplus during warm season commonly occurs in the case of micro-chp, when the plant can be operated close to constant power only and shutdowns are not preferred (e.g. SOFC plant) Significant thermal losses, poor annual efficiency Solution: seasonal thermal storage

17 Thermal storage technologies in a nutshell Mass storages: Principle: heat flow warms up a mass (e.g. tank of water) Key features: good availability and thermal capacity simple structure only applicable in low temperatures (< 100ºC) Market status: commercial technology Phase change materials (PCM): Principle: thermal energy stored during the evaporation of melting of a material Key features: large storage capacity at constant temperature early phase of development, expensive Market status: emerging technology Thermo-chemical energy storage Operational principle: thermal energy stored during fuel synthesis (e.g. ammonia dissociation: 2NH 4 +heat 4H 2 +N 2 ; takes place in approximately 250ºC) Key features: large storage capacity and density no significant storage losses early phase of development, expensive Market status: experimental technology Applicability of seasonal thermal storages Operational environment: climatic conditions, e.g. ground temperature, snowcovered ground geological structure of the building site Inlet temperatures of the heating system: 40ºC (low temperature heating system) 70ºC (conventional radiator heating in Finland) Trade-off between storage capacity and storage losses must be found!

18 Integration of seasonal thermal storage into residential micro-chp plant Buffer storage ) (>100ºC) Heat pump Heat exchanger Electrical storages in a nutshell Basic requirements: large charge-discharge quantities must tolerate high discharge power minor service requirements safety longevity high energy density Other methods (examples): (super) capacitors mechanical storages (flywheels, compressed air etc.) Selected batteries: Lead-acid- battery good availability at low price (4-6 Wh/ ) low energy density Wh/L Lithium-ion-battery in the market, medium price (1.5-2 Wh/ ) very high energy density Wh/L low self-discharge NiMH- battery in the market, high price (1 Wh/ ) high energy density Wh/L high self-discharge LiFePO 4 - battery emerging, high price (< 1Wh/ ) high energy density 170 Wh/L low service requirement For self-learning: Chen, H., Cong, T.N., Yang, W., Tan, C., Li, Y. and Ding, Y. (2009) Progress in electrical energy storage system: A critical review. Progress in Natural Science 19(3):

19 Electricity to the grid? Monetary compensation for the electricity fed into the grid may be based on: Feed-in tariffs the utilities are obliged to buy electricity from small producers at rates set by the government (buyback rate) a two-directional electricity metering required applied in many European countries Net-metering the deduction of energy outflows from metered energy inflows and compensated through a retail credit by a utility Time-of use metering Two-directional metering strategy that allows rate schedule depending on the peak demand hours The stability of the grid limits the amount of grid-connected small-scale producers Trends of development Polygeneration simultaneous production of electricity heating and cooling energy (at various enthalpy levels) fuel synthesis (e.g. hydrogen, methane) desalination dehumidification Hybrid systems micro-cogeneration technology + another micro-cogeneration technology solar thermal/pv/micro-wind heat pump energy storage

20 Example: Trigeneration: electricity + thermal + cooling Giovanni Angrisani, G., Minichiello, F., Roselli, C. and Sasso, M. (2010) Desiccant HVAC system driven by a micro-chp: Experimental analysis. Energy and Buildings 42 (11) : Example: Hybrid: SOFC + MT Park, S.K., Oh, K.S., Kim, T.S. (2007) Analysis of the design of a pressurized SOFC hybrid system using a fixed gas turbine design. Journal of Power Sources 170(1):

21 Literature Pehnt, M., Cames, M., Fischer, C., Praetorius, B., Schneider, L., Schumacher, K., Voß, J.-P. (2006) Micro Cogeneration: Towards Decentralized Energy Systems. 346 p. ISBN: Knowles, M. and Burdon, I. (2004) Micro Energy Systems: Review of Technology, Issues of Scale and Integration, John Wiley and Sons, 180 p. ISBN Internet: