1.0

0.8

0.6

m/md

0.4

0.2

0

2190

4380

6570

8760

t (hr)

28 is an even function about its midpoint of 8760/2 = 4380 hours, the number of hours

for which the load will exceed any given level is simply twice the number of hours

from the time of maximum load (*t *= 0) until the time that load would have occurred.

This yields an equation of the same form as eq 2-28, except that the period of the

function is now twice as long, i.e.,

˙ ˙

(2-29)

The form of the resulting load duration curve is shown as Figure 2. This result is

similar to the shape of empirical load curves determined by other investigators, such

as Werner (1984), except that it over-predicts the number of hours at which high

loads occur. Since this will result in a conservative design, this is a suitable

approximation for design purposes. The equivalent full load utilization time is

another factor used to evaluate the load in district heating systems. The equivalent

full load utilization time is the amount of time at which the load would have to be

at its maximum value to result in the total heat supplied being equal to that supplied

over the actual yearly cycle. This time can be easily found by integrating eq 2-28 over

the yearly cycle

8760

∫

(0.425 cos (2π *t*/8760) + 0.575) d *t*

(2-30)

0

yields a value of *t*u = 5037 hours. This value is near the upper end of the range of

measured values reported by Bhm (1988) for a number of Danish district heating

systems.

A number of other technical parameters need to have values assigned to them for

us to proceed with this sample calculation. The following values selected are felt to

be well within the range of what could reasonably be expected in an actual design:

14

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