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2.2 Cooling Project Load and Energy Calculation
Cooling load and energy calculation is treated very much the same way as heating load and energy use, with a few variations as explained below.
Cooling load and energy calculation is treated very much the same way as heating load and energy use, with a few variations as explained below.
2.2.1 Site climatic conditions
As in the heating case, site climatic conditions use the concepts of design temperature and degree-days – this time design cooling temperature and cooling degree-days. The former represents an exceptionally warm day and corresponds to the maximum temperature that has been measured for a frequency level of at least 1% over the year, for a specific area [ASHRAE, 1997]; it is useful to determine cooling loads. The latter is useful to determine cooling needs and is defined as the sum of daily differences between the average daily temperature and a set temperature Tset (10°C in the RETScreen CHP model). Mathematically:
As in the heating case, site climatic conditions use the concepts of design temperature and degree-days – this time design cooling temperature and cooling degree-days. The former represents an exceptionally warm day and corresponds to the maximum temperature that has been measured for a frequency level of at least 1% over the year, for a specific area [ASHRAE, 1997]; it is useful to determine cooling loads. The latter is useful to determine cooling needs and is defined as the sum of daily differences between the average daily temperature and a set temperature Tset (10°C in the RETScreen CHP model). Mathematically:
where CDDi is the monthly cooling degree-days for month i , Ni is the number of days in month i , and Ta,k is the average daily temperature for day k of the month. The annual degree-days CDD is calculated by adding the monthly degree-days:
Again, the main advantage of using degree-days is that in first approximation, the cooling needs of a building can be assumed to be proportional to the number of cooling degree-days.
2.2.2 Equivalent degree-days for base load cooling
Base load cooling represents non-weather dependent process cooling needs such as internal heating loads or constant cooling loads. The base load cooling is entered by the user as a fraction d of the annual space cooling energy use (including base load cooling, but excluding process cooling energy use).
The method used in section 2.1.2 can be used again to define equivalent cooling degree-days for base load cooling. The equations are identical, with the base cooling use replacing the domestic hot water demand and cooling degree-days replacing heating degree-days. The final result is:
Base load cooling represents non-weather dependent process cooling needs such as internal heating loads or constant cooling loads. The base load cooling is entered by the user as a fraction d of the annual space cooling energy use (including base load cooling, but excluding process cooling energy use).
The method used in section 2.1.2 can be used again to define equivalent cooling degree-days for base load cooling. The equations are identical, with the base cooling use replacing the domestic hot water demand and cooling degree-days replacing heating degree-days. The final result is:
where cddCL is the equivalent cooling degree-days corresponding to the base cooling load. Here again the model takes into account constant cooling loads in a rather coarse way. The model assumes that this load is constant for every day of the year. In a cold climate the cooling might be completely turned off during the heating season and the factor d should be set to zero.
2.2.3 Calculation of peak space cooling load
The space cooling load is entered directly by the user as a value in Watts per square metre of cooled floor area. Formulae (9) and (10) have their direct equivalents for cooling:
The space cooling load is entered directly by the user as a value in Watts per square metre of cooled floor area. Formulae (9) and (10) have their direct equivalents for cooling:
where Psc,j is the total peak cooling load for the jth cluster of buildings, psc,j is the peak cooling load per unit area entered by the user, Aj is the total cooled area of the jth cluster of buildings. The total peak space cooling load for all the clusters of buildings Psc seen by the system is:
where the summation is done on all clusters of buildings. Details about the estimation of the peak cooling load per unit area can be found in section 2.6.2.
2.2.4 Cooling load duration curve
A cooling load duration curve can be derived using a method similar to the one described in section 2.1.4 for heating. The only difference is that the empirical coefficients F1, F2 ,...,F12 will be different. In the RETScreen CHP cooling model the coefficients are all set to one, which is equivalent to say that the heating load for all months is directly proportional to the number of cooling degree days (as shown by equations (13-0) to (13-13)). The rest of the calculation is similar to the heating case.
A cooling load duration curve can be derived using a method similar to the one described in section 2.1.4 for heating. The only difference is that the empirical coefficients F1, F2 ,...,F12 will be different. In the RETScreen CHP cooling model the coefficients are all set to one, which is equivalent to say that the heating load for all months is directly proportional to the number of cooling degree days (as shown by equations (13-0) to (13-13)). The rest of the calculation is similar to the heating case.
2.2.5 Peak load period, total energy use, etc.
As was done for the heating load analysis in section 2.1.5, a peak load period is added to represent the time of year where the cooling system works at full capacity. The rest of the calculation of the cooling use is in all points similar to that of the heating use, with cooling loads replacing heating loads in all equations.
As was done for the heating load analysis in section 2.1.5, a peak load period is added to represent the time of year where the cooling system works at full capacity. The rest of the calculation of the cooling use is in all points similar to that of the heating use, with cooling loads replacing heating loads in all equations.
2.2.6 Process cooling
Process cooling is treated exactly the same way as process heating. Formulae similar to equations (17) to (19) can be written, except that they apply to cooling processes rather than heating processes.
Process cooling is treated exactly the same way as process heating. Formulae similar to equations (17) to (19) can be written, except that they apply to cooling processes rather than heating processes.
