Patent Publication Number: US-2012041610-A1

Title: Controller for combined heat and power system

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority to U.S. Provisional Patent Application Ser. No. 61/173,792 entitled “Controller for Combined Heat and Power System” filed on Apr. 29, 2009. The content of this application is incorporated herein by reference in its entirety. 
    
    
     GOVERNMENT CONTRACT 
     The disclosure described herein was made during the course of or in the performance of work under U.S. Government Contract No. 4000009518(17) awarded by the Department of Energy. 
    
    
     FIELD OF THE DISCLOSURE 
     This disclosure is related generally to power conversion systems, and more specifically to a controller for controlling a combined heat and power system to optimize energy production costs. 
     BACKGROUND OF THE DISCLOSURE 
     Combined heat and power (CHP) systems are widely employed to provide facility electricity, heating and cooling for commercial, industrial or residential sites. A typical CHP system produces heat by combusting fuel, then transforms the heat into mechanical power using, e.g., a turbine, and finally transforms the mechanical power into electric power using, e.g., a generator. The thermal energy in the exhaust from the turbine is used to provide useful heating thermal output. A CHP system can also include an absorption chiller to produce cooling thermal output. Examples include a fuel cell power plant for producing electricity and useful heating thermal output, and an absorption chiller hybrid for producing electricity and useful cooling or heating power output. 
     Proper coordination of different types of CHP systems is required during operation to realize projected benefits in operating costs. Conventional modes of operation, such as load following, peak shaving, and base loading may not achieve full savings in markets with large price variability. It is often difficult for a human operator to choose the correct mode of operation and/or correct operational parameters. Thus, a need exists to provide means and methods of automatically controlling a CHP system to optimize the energy production costs. 
     SUMMARY 
     There is provided a controller for controlling a combined heat and power (CHP) system, which can include one or more CHP units. Each CHP unit can be characterized by a CHP unit type, and can be configured to generate heating, cooling and/or electric power. The controller according to the present invention can comprise a high level optimizer and one or more low level optimizers. 
     The high level optimizer can be configured to optimize a total cost of producing heating, cooling, and electric power, by allocating total heating, cooling, and/or electric power setpoints to at least one CHP unit type, based on the fuel price, CHP unit operational constraints, and/or heating, cooling, and/or electric power demand. 
     The low level optimizer can be configured to allocate cooling, heating, and/or electric power setpoints to individual CHP units, based on the high level allocation to CHP unit types. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an example of hierarchical architecture of a CHP system controller according to the present disclosure. 
         FIG. 2  illustrates a component view of one embodiment of a CHP unit. 
         FIG. 3  illustrates input/output views of several CHP unit types. 
         FIG. 4  illustrates an example of a method of load distribution among individual CHP units implemented by a low level optimizer according to the present disclosure. 
     
    
    
     The drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the present invention. In the drawings, like numerals are used to indicate like parts throughout the various views. 
     DETAILED DESCRIPTION OF THE DISCLOSURE 
     There is provided a controller for controlling a combined heat and power (CHP) system. The CHP system can combust fuel and produce useful electric power output, as well as useful heating and/or cooling power output. 
     In one aspect, the controller  100  according to the present disclosure can have a hierarchical architecture comprising a high level optimizer  110  and one or more low level optimizers  120 , as best viewed in  FIG. 1 . The high level optimizer  110  can optimize the total cost of producing useful energy outputs, including heating, cooling, and electric energy. The low level optimizer  120  can distribute the load among individual CHP units. 
     In one embodiment, the CHP system  150  controlled by the controller  100 , can be employed to satisfy the demands of the consumer  160  (e.g., a commercial, industrial or residential building) in heating, cooling, and/or electric power. The demand levels can be pre-determined, or can vary, e.g., depending on the time of day. 
     In another aspect, an auxiliary heating and/or cooling system can be provided at the consumer site. In a further aspect, the consumer site can be connected to an electric grid  180  and be able to import electricity from the grid when it is economically favorable or in order to satisfy the demand for electricity which can not be satisfied by the CHP system  150  (e.g., a peak demand). 
     In one embodiment, the pre-processing block  130  can compute the demand levels for electric and thermal power and output the computed demand levels to the high level optimizer  110 . In another aspect the pre-processing block  130  can supply to the high level optimizer  110  other information necessary for controller functioning, including, e.g., grid electricity pricing information. 
     In one embodiment, the CHP system  150  can include one or more CHP units. In one embodiment, a CHP unit  200  can include, as best viewed in  FIG. 2 , one or more electricity sources  210   a - 210   z  (e.g., a gas turbine or a steam generator), one or more heating sources  220   a - 220   z  (e.g., a boiler), and/or one or more cooling sources  230   a - 230   z  (e.g., an absorption chiller). 
     In another aspect, the CHP system  150  of  FIG. 1  can include one or more CHP units of different CHP unit types. In one embodiment, the CHP system can include one or more PureComfort® CHP units, one or more PureComfort Trigen® CHP units, and one or more PureThermal® CHP units. 
       FIG. 3  illustrates models of several CHP unit types showing the input and output signals reflecting the basic functionality. A PureComfort® CHP unit is a microturbine-absorption chiller hybrid producing electricity and useful cooling or heating power output, which can be configured to operate in cooling mode  320   a  or heating mode  320   b . A PureComfort Trigen® CHP unit  340  can produce electricity and cooling and heating thermal outputs. A PureThermal® CHP unit  330  can produce electricity and useful heating thermal output. A skilled artisan would appreciate the fact that other types of CHP units are within the scope and the spirit of the invention. 
     The cost of energy production by a CHP system can depend on a number of variables, including the price of fuel, the price of electricity imported from a grid, and operational characteristics of CHP units. 
     In one embodiment, the total cost of producing useful energy output can be calculated as follows: 
     
       
         
           
             
               
                 
                   
                     Cost 
                      
                     
                       ( 
                       
                         $ 
                         hour 
                       
                       ) 
                     
                   
                   = 
                   
                     
                       
                         
                           F 
                            
                           
                               
                           
                            
                           P 
                         
                         
                           Eta 
                           MT 
                         
                       
                       · 
                       
                         P 
                         PC 
                       
                     
                     + 
                     
                       
                         FP 
                         
                           Eta 
                           MT 
                         
                       
                       · 
                       
                         p 
                         PCh 
                       
                     
                     + 
                     
                       
                         
                           FP 
                           
                             Eta 
                             MT 
                           
                         
                         · 
                         
                           
                             P 
                             PCT 
                           
                           ++ 
                         
                       
                        
                       
                         
                           FP 
                           
                             Eta 
                             MT 
                           
                         
                         · 
                         
                           P 
                           PCTh 
                         
                       
                     
                     + 
                     
                       
                         FP 
                         
                           Eta 
                           MT 
                         
                       
                       · 
                       
                         P 
                         PT 
                       
                     
                     + 
                     
                       
                         FP_Aux 
                         
                           Eta 
                           aux_boiler 
                         
                       
                       · 
                       
                         H 
                         aux 
                       
                     
                     + 
                     
                       EP 
                       · 
                       
                         P 
                         grid 
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     wherein FP is the fuel price, 
     Eta mt  is the net electrical efficiency of the microturbine; 
     P PC  is the electric power output of the PureComfort® CHP units; 
     P PCH  is net electric power output of the PureComfort® CHP unit configured in heating mode; 
     P PCT  is the net electric power output of the PureComfort Trigen® CHP units; 
     P PCTH  is net electric power output of the PureComfort Trigen® CHP unit configured in heating mode; 
     P PT  is the electric power output of the PureThermal® CHP units; 
     FP_Aux is the fuel price for the auxiliary heater (one or more heating sources defined as being external to CHP system considered, such as a boiler); 
     H aux  is the heating power output from the auxiliary heaters (a heating source defined as being external to CHP system considered, such as a boiler); 
     Eta aux     —     boiler  is the efficiency of the auxiliary heater; 
     EP is the price of electricity imported from grid ($/kWh); 
     P grid  is the amount of electricity imported from grid. 
     In another aspect, an operating envelope for each of the CHP unit types can be described by a set of inequality and/or equality constraints in the C, H, P space, where C is the cooling power output, H is the heating power output, and P is the electric power output of the CHP unit. For example, an operating envelope for a PureComfort® CHP unit can be described by the following set of linear inequality constraints in the cooling mode (2) and the heating mode (3) of operation: 
         P   PC +α 1   C   PC ≦α 2   (2)
 
         P   PCh +α 1h   H   PCh ≦α 2h   (3)
 
     wherein α 1  is a pre-determined constant value derived from experiments or from equipment specifications;
 
α 2  is a pre-determined constant value derived from experiments or from equipment specifications;
 
α 1h  is a pre-determined constant value derived from experiments or from equipment specifications;
 
α 2h  is a pre-determined constant value derived from experiments or from equipment specifications;
 
C PC  is the cooling power output by a PureComfort® CHP unit; and
 
H PCh  is the heating power output by a PureComfort® CHP unit.
 
     An operating envelope for a PureComfort Trigen® CHP unit can be described by the following set of linear inequality constraints in the cooling mode (4)-(5) and the heating mode (6) of operation: 
         P   PCT +β 1   C   PCT +β 2   H   PCT ≦β 3  
 
       β 4   C   PCT   +H   PCT ≦β 5   (4)-(5)
 
     wherein 
     β 1  is a pre-determined constant value derived from experiments or from equipment specifications;
 
β 2  is a pre-determined constant value derived from experiments or from equipment specifications;
 
H PCT  is the heating power output from the PureComfort Trigen® CHP unit;
 
β 3  is a pre-determined constant value derived from experiments or from equipment specifications;
 
β 4  is a pre-determined constant value derived from experiments or from equipment specifications;
 
β 5  is a pre-determined constant value derived from experiments or from equipment specifications;
 
         P   PCTh +β 1h     —     PCT   H   PCTh ≦β 2h     —     PCT   (6)
 
     wherein 
     β 1h     —     PCT  is a pre-determined constant value derived from experiments or from equipment specifications;
 
H PCTh  is the heating power output by PureComfort Trigen® CHP unit configured in heating-only mode; and
 
β 2h     —     PCT  is a pre-determined constant value derived from experiments or from equipment specifications;
 
     An operating envelope for a PureThermal® CHP unit can be described by the following linear equality constraint: 
         P   PT +γ 1     —     PT   H   PT =γ 2     —     PT   (7)
 
     wherein 
     γ 1     —     PT  is a pre-determined constant value derived from experiments or from equipment specifications;
 
H PT  is the heating power output by a PureThermal® CHPunit; and
 
γ 2     —     PT  is a pre-determined constant value derived from experiments or from equipment specifications;
 
     In a further aspect, the high level optimizer can include additional equality constraints reflecting the matching of power supply to power demand: 
     
       
         
           
             
               
                 
                   
                     
                       
                         P 
                         PC 
                       
                       + 
                       
                         P 
                         PCh 
                       
                       + 
                       
                         P 
                         PCT 
                       
                       + 
                       
                         P 
                         PCTh 
                       
                       + 
                       
                         P 
                         PT 
                       
                       + 
                       
                         P 
                         grid 
                       
                     
                     = 
                     
                       
                         P 
                         load 
                       
                       + 
                       
                         
                           1 
                           
                             COP_aux 
                              
                             _chiller 
                           
                         
                         · 
                         
                           C 
                           aux 
                         
                       
                     
                   
                    
                   
                     
 
                   
                    
                   
                       
                   
                    
                   
                     
                       
                         C 
                         PC 
                       
                       + 
                       
                         C 
                         PCT 
                       
                       + 
                       
                         C 
                         aux 
                       
                     
                     = 
                     
                       C 
                       load 
                     
                   
                    
                   
                     
 
                   
                    
                   
                       
                   
                    
                   
                     
                       
                         H 
                         PCh 
                       
                       + 
                       
                         H 
                         PCT 
                       
                       + 
                       
                         H 
                         PCTh 
                       
                       + 
                       
                         H 
                         PT 
                       
                       + 
                       
                         H 
                         aux 
                       
                     
                     = 
                     
                       H 
                       load 
                     
                   
                 
               
               
                 
                   
                     ( 
                     8 
                     ) 
                   
                    
                   
                     - 
                   
                    
                   
                     ( 
                     10 
                     ) 
                   
                 
               
             
           
         
       
     
     wherein 
     P load  is the electric power demand to be satisfied by the CHP system; 
     C load  is the cooling power demand to be satisfied by the CHP system; 
     H load  is the heating power demand to be satisfied by the CHP system; 
     COP_aux_chiller is the “coefficient of performance” (ratio of cooling power output to electrical power input) of the auxiliary chiller; 
     C aux  is the cooling power output of the auxiliary chiller (one or more chillers defined to be external to the CHP system being considered, such as an electrical chiller); 
     In a further aspect, the cost function can further include additional decision variables reflecting electricity demand charges which can be modeled based on the maximum power consumption in a predetermined period of time (e.g., a month), as well as the maximum power consumption during the peak, part-peak, and off-peak periods. Hence, the cost function can be defined as follows: 
     
       
         
           
             
               
                 
                   
                     Cost 
                      
                     
                       ( 
                       
                         $ 
                         hour 
                       
                       ) 
                     
                   
                   = 
                   
                     
                       
                         FP 
                         
                           eta 
                           MT 
                         
                       
                       · 
                       
                         P 
                         PC 
                       
                     
                     + 
                     
                       
                         FP 
                         
                           eta 
                           MT 
                         
                       
                       · 
                       
                         P 
                         PCh 
                       
                     
                     + 
                     
                       
                         
                           FP 
                           
                             eta 
                             MT 
                           
                         
                         · 
                         
                           
                             P 
                             PCT 
                           
                           ++ 
                         
                       
                        
                       
                         
                           FP 
                           
                             eta 
                             MT 
                           
                         
                         · 
                         
                           P 
                           PCTh 
                         
                       
                     
                     + 
                     
                       
                         FP 
                         
                           eta 
                           MT 
                         
                       
                       · 
                       
                         P 
                         PT 
                       
                     
                     + 
                     
                       
                         FP_Aux 
                         
                           eta 
                           aux_boiler 
                         
                       
                       · 
                       
                         H 
                         aux 
                       
                     
                     + 
                     
                       
                         EP 
                         · 
                         
                           
                             P 
                             grid 
                           
                           ++ 
                         
                       
                        
                       
                         EP_Dem 
                         · 
                         delta_Pmax 
                       
                        
                       
                         _dem 
                         ++ 
                       
                        
                       EP_Dem 
                        
                       
                         _period 
                         · 
                         delta_Pmax 
                       
                        
                       _dem 
                        
                       _period 
                     
                   
                 
               
               
                 
                   ( 
                   11 
                   ) 
                 
               
             
           
         
       
     
     wherein 
     EP_Dem is the electricity demand price over a billing period such as a month, $/kW; delta_Pmax_dem is the change in the maximum power drawn from the grid over the current billing cycle computed by the high-level optimizer at each sampling time; EP_Dem_period is the electricity demand price applicable to “period” (where period represents peak, part-peak or off-peak intervals of a day) within the current billing period, $/kW;
 
Delta_Pmax_dem_period is the change in the maximum power drawn from the grid in “period” (where period represents peak, part-peak or off-peak intervals of a day) within the current billing cycle computed by the high-level optimizer at each sampling time;
 
     The additional inequality constraints to reflect electricity demand charges can include: 
       delta —   P max —   dem ≧0
 
       delta —   P max —   dem _period≧0
 
         P   grid −delta —   P max —   dem≦P _max
 
         P   grid −delta —   P max —   dem _period≦ P _period  (12)-(15)
 
     wherein 
     P_max is the maximum power drawn from the grid over the current billing period;
 
P_period is the maximum power drawn from the grid during “period” (where period represents peak, off-peak or part-peak intervals of a day) within the current billing period;
 
     Thus, in one embodiment, the high level optimizer can optimize the total cost of producing heating, cooling, and/or electric power, by allocating heating, cooling, and/or electric power setpoints to one or more CHP unit types, based on one or more of the following inputs: fuel price, power output demands for heating, cooling, and/or electric power, operational constraints for one or more of CHP unit types, price of electric power imported from the grid, and electricity demand charges. In a further aspect, the high level optimizer can perform the optimization at least once in a control interval which can be a pre-determined period of time (e.g., one hour). A skilled artisan would appreciate the fact that other control interval values, as well as performing the high level optimization responsive to an event, are within the scope and the spirit of the present invention. 
     In a further aspect, the cost function and the constraints can be linear, affine or nonlinear, and thus the high level optimizer can optimize the cost function using linear or nonlinear programming methods known in the art, e.g., an interior point method described in the book, Convex Optimization, by Stephen Boyd (Cambridge University Press, 2004, ISBN: 0521833787). 
     In another aspect, a low level optimizer can receive from the high level optimizer the electric power, cooling, and/or heating setpoints for one or more CHP unit types (e.g., PureComfort®, PureConfort Trigen®, and PureThermal®), and distribute the load among individual CHP units within each CHP unit type. The low level optimizer can further provide run-time balancing where-in the difference between the run-times of various microturbines over a long enough period (eg, quarters to years) is minimized by prioritizing what units can be turned off or on next, ensuring the minimum on-time and off-time constraints for various units (to ensure that the inefficiency during startup &amp; shutdown does not negate the cost savings anticipated from the optimal supervisory control strategy), and minimizing the risk of power export to grid due to load changes between the sampling instances of the high level optimizer in situations where power export to the grid is not permitted. This constraint, for example, can be handled by maintaining a margin in the power drawn from the grid and by having a hardware protection in place for redundancy. 
     In a further aspect, the electric, heating and cooling setpoints can be treated independently by the low level optimizer. The load allocation by the low level optimizer can be characterized by one or more scheduling rules which can be implemented, e.g., as a decision tree an example of which is presented in  FIG. 4 . In one embodiment, the scheduling rules can be determined off-line, thus significantly reducing the requirements to the processing power of the low level optimizer. 
       FIG. 4  illustrates an example of a method of load distribution among individual CHP units implemented by a low level optimizer. 
     At step  410 , the method selects the minimum among the sum of power setpoints received from the high level optimizer and the maximum power that can be produced by the CHP system. The latter can be determined by the equipment level controller or the calculation can be performed within the lower level optimizer using equipment performance data from suppliers &amp; other site specific information and online measurements such as the altitude and ambient temperature. Although the high level optimizer takes into account the maximum limits on each class of CHP equipment, this is an example of additional and redundant protection that can be implemented in the lower level optimizer The processing continues at step  420 . 
     At step  420 , the method determined the number of master microturbines by bracketing power, that is, by identifying the minimum number of microturbines needed to produce the required power. The processing continues at step  430 . 
     At step  430 , the method ascertains whether the number of running master microturbines is equal to the required number computed in step  420  above and if so, the processing continues at step  460 ; otherwise the method branches to step  440 . 
     At step  440 , the method ascertains whether the required number of master microturbines can be turned on or off, based on on-time and off-time constraints for an individual microturbine, and if so, the processing continues at step  450 ; otherwise, the method branches to step  460 . 
     At step  450 , the low level optimizer turns the required number of master microturbines on or off. The processing continues at step  460 . 
     At step  460 , the load of each running microturbine is computed, e.g., by using an optimal loading strategy within the pack, which can be, for example, an “even loading” of all the microturbines or “staging or max-loading” where all but one of the microturbines run at or close to full-power. The processing continues at step  470 . 
     At step  470 , the method ascertains that the load on all running master microturbines is greater than a predetermined minimum load value, and if so, the method branches to step  490 ; otherwise, the processing continues at step  480 . 
     At step  480 , the low level optimizer reduces the power setpoint for maximally loaded master microturbines by a predetermined value, and adds the predetermined value to the master microturbines with low load, in order to ensure that the minimum power constraint is satisfied on all master microturbines. The processing continues at step  490 . 
     At step  490 , the low level optimizer dispatches the computed power setpoints to individual CHP units, and the method terminates. 
     In another aspect, the low level optimizer  120  of  FIG. 1  can perform computing the power setpoints to individual CHP units responsive to receiving setpoints per CHP unit types from the high level optimizer. A skilled artisan would appreciate the fact performing the low level optimization responsive to other events, including a predetermined timeout expiration, is within the scope and the spirit of the present invention. 
     In another aspect, the post-processing block  140  of  FIG. 1  can transform the setpoints outputted by the low level optimizer into a format suitable to be supplied to the actual CHP units.