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2.5 Energy Delivered and Fuel Consumption
Now that the heating, cooling and power loads and use have been defined (sections 2.1, 2.2 and 2.3) and that the characteristics of the CHP equipment are known (section 2.4), it becomes possible to calculate how loads and energy use are met by the base, intermediate and peak systems. This is described in section 2.5.1. Power generating system availability and operating strategy considerations are treated in sections 2.5.2 and 2.5.3, respectively.
Now that the heating, cooling and power loads and use have been defined (sections 2.1, 2.2 and 2.3) and that the characteristics of the CHP equipment are known (section 2.4), it becomes possible to calculate how loads and energy use are met by the base, intermediate and peak systems. This is described in section 2.5.1. Power generating system availability and operating strategy considerations are treated in sections 2.5.2 and 2.5.3, respectively.
2.5.1 Load met by the base, intermediate and peak systems
The average monthly and peak loads for heating, cooling and power generation, as determined in sections 2.1 to 2.3 , are met by base, intermediate, and peak load systems. Typically, the lowest cost energy is used to produce the base load. Then the intermediate load system, if applicable, is dispatched to meet most of the rest of the energy needs. Finally, the peak load system meets the top portion of the annual energy needs during peak periods. The fraction of the total needs met by each system depends on their respective sizes; for example the heating load of Figure 5 can be met by a waste heat recovery system (base), a biomass heating system (intermediate), and a gas-fired boiler (peak). This is illustrated in Figure 14. In this figure, the base heating system can meet 10% of the peak load, the intermediate heating system meets another 30%. Together these two systems meet the heating load for all months except during the months of December, January and February and during the peak period. The boiler supplies the rest of the load during those times.
The average monthly and peak loads for heating, cooling and power generation, as determined in sections 2.1 to 2.3 , are met by base, intermediate, and peak load systems. Typically, the lowest cost energy is used to produce the base load. Then the intermediate load system, if applicable, is dispatched to meet most of the rest of the energy needs. Finally, the peak load system meets the top portion of the annual energy needs during peak periods. The fraction of the total needs met by each system depends on their respective sizes; for example the heating load of Figure 5 can be met by a waste heat recovery system (base), a biomass heating system (intermediate), and a gas-fired boiler (peak). This is illustrated in Figure 14. In this figure, the base heating system can meet 10% of the peak load, the intermediate heating system meets another 30%. Together these two systems meet the heating load for all months except during the months of December, January and February and during the peak period. The boiler supplies the rest of the load during those times.
Figure 14: Parts of the Heating Load Met by Various Energy Systems
The calculation of the load met by the base, intermediate and peak load systems will be first illustrated in the case of the heating systems. The case of cooling system is very similar. The calculation of the power generating system is slightly more complicated because of the minimum capacity at which the system has to be run.
Heating system
Let
H,i be the average heating load for period i ( i ranges from 1 to 12 for individual months and is equal to 13 for the peak period), and let PBaseH , PInterH and PPeakH be the capacities of the base, intermediate and peak heating systems. The energy needs QBaseH,i met by the base heating system for period i is:
Let
where Ňi is the corrected number of hours in the period i (see Table 4). The load not met by the base heating system for that period i is simply:
The energy needs QInterH,i met by the intermediate heating system for period i is then:
The load not met by the either the base of the intermediate heating systems for that period i is then:
and finally the energy needs QPeakH,i met by the peak heating system during period i is:
(in this last case, the peak load is not met and a warning is displayed to the user). The heating use delivered by the base, intermediate and peak heating systems over the year is obtained simply by summing the contributions of all 13 periods.
Power generating system
In the case of a power generating system, the turndown of cogeneration equipment sometimes limits the operating conditions. For example if mBasep is the minimum capacity of the base power system under consideration, then the energy needs QBasep,i it meets during period i is:
Power generating system
In the case of a power generating system, the turndown of cogeneration equipment sometimes limits the operating conditions. For example if mBasep is the minimum capacity of the base power system under consideration, then the energy needs QBasep,i it meets during period i is:
where Pp,i is the power load for period i ( i =1,…,13), PBasep is the capacity of the base power system, and Ňi is the corrected number of hours in period i (see Table 4). Subsequent calculations, such as (51), are automatically adjusted for the change in the value of QBasep,i.
2.5.2 Power generating system availability
For the base and intermediate power generating systems, the user enters the estimated availability in hours or percent of the year. The model calculates the electricity (and co-generated heat) delivered based on the availability. The amount of energy that is not produced by the base load system, because the availability is less than 100%, needs to be produced by the intermediate and/or peak load systems. Heating and cooling systems have an assumed availability of 100%.
Typical values for availability expressed in hours for a new generating system are between 8,000 (91.3%) and 8,400 hours (95.9%) per year. The values may be lower for used and older equipment. If down time can be scheduled to low load times the availability hours can be increased as the model deals with the average monthly load.
The model assumes that the down time is spread equally over the year; so if aBasep is the fraction of time that the base power generating system is available, the energy delivered by the base system for every period, as calculated by (55), has to be multiplied by aBasep . Subsequent calculations, such as (51), are again automatically adjusted for the change in the value of QBasep,i . Note that the amount of co-generated heat that can be reclaimed varies in the same proportion.
For the base and intermediate power generating systems, the user enters the estimated availability in hours or percent of the year. The model calculates the electricity (and co-generated heat) delivered based on the availability. The amount of energy that is not produced by the base load system, because the availability is less than 100%, needs to be produced by the intermediate and/or peak load systems. Heating and cooling systems have an assumed availability of 100%.
Typical values for availability expressed in hours for a new generating system are between 8,000 (91.3%) and 8,400 hours (95.9%) per year. The values may be lower for used and older equipment. If down time can be scheduled to low load times the availability hours can be increased as the model deals with the average monthly load.
The model assumes that the down time is spread equally over the year; so if aBasep is the fraction of time that the base power generating system is available, the energy delivered by the base system for every period, as calculated by (55), has to be multiplied by aBasep . Subsequent calculations, such as (51), are again automatically adjusted for the change in the value of QBasep,i . Note that the amount of co-generated heat that can be reclaimed varies in the same proportion.
2.5.3 Operating strategy
The user selects the operating strategy for the base or intermediate load system as either Full power capacity output, Power load following, or Heating load following. When the operating strategy is set to follow either heating or power load, and the load required is less than produced for 100% output but more than then minimum capacity of the generating equipment, the output of the generating equipment will be reduced according to:
The user selects the operating strategy for the base or intermediate load system as either Full power capacity output, Power load following, or Heating load following. When the operating strategy is set to follow either heating or power load, and the load required is less than produced for 100% output but more than then minimum capacity of the generating equipment, the output of the generating equipment will be reduced according to:
where mi is the capacity in percent required to meet the load in period i , Pp is the capacity of the generating equipment and Pp,i,m is the reduced output to meet the load in the period i. Both heating and power output will be reduced on a linear basis. Improved or reduced performance on part load is not calculated for any type of equipment.
Note finally that when more heat is available than required to meet the heating load, this heat is assumed disposed by a cooling system.
Note finally that when more heat is available than required to meet the heating load, this heat is assumed disposed by a cooling system.
