Simultaneity and Part Load

Overview

Not all consumers of a heat network reach their peak load at the same time. VICUS Districts represents this through two separate quantities: simultaneity (diversity) reduces the consumer loads aggregated along shared routes, and the part-load fraction reduces the load actually drawn by an individual consumer. Simultaneity is only taken into account if it is activated for the network; it then acts on pipe sizing, steady-state calculation and dynamic simulation. The part-load fraction is applied independently of this - even when simultaneity is deactivated - and acts exclusively on the steady-state calculation and dynamic simulation, not on pipe sizing.

This page explains the calculation model. Its operation is described on two follow-up pages:

• the network properties tab Simultaneity and Part Load
• the dialog Simultaneity Calculation

What does the simultaneity factor mean?

The simultaneity factor of a route is the ratio of the peak load actually occurring simultaneously to the sum of the individual peak loads of all consumers supplied via that route:

$$g_\text{route} = \frac{\text{simultaneous peak load of the route}}{\sum_i \dot{Q}_{\text{peak},i}}$$

A value of 1.0 means full simultaneity - all consumers peak together, there is no relief effect. Values smaller than 1.0 indicate that the peaks occur at different times and the route only has to carry a fraction of the summed individual peaks. The physical floor of the default curve is at about 0.45 (see below).

Target simultaneity curve

For each route the calculation targets a simultaneity factor that depends on the number $n$ of downstream consumers. The default curve follows Winter et al. (Euroheat & Power, 2001):

$g(n) = \min\left(1,\; a + \frac{b}{1 + \left(n/c\right)^{d}}\right)$

with $a = 0.4497$, $b = 0.5512$, $c = 53.84$ and $d = 1.7627$. The curve is stored as a lookup table up to a consumer count of about 300 (support points $n = 1, 2, 3, 4, 5, 10, 15, \dots, 295$, linearly interpolated in between). It decreases monotonically with rising consumer count from 1.0 (single consumer) towards the floor $a \approx 0.45$.

The target curve applies network-wide. For individual pipes it can be overridden by a custom simultaneity, and its support points can be edited in the dialog Simultaneity Calculation.

Time shift of the demand profiles

The achieved simultaneity factor does not result from a mere multiplication but from an optimization: each consumer profile is shifted forward or backward within a maximum permissible time shift, so that the peak loads on shared routes are spread out. The optimizer searches for the shifts that bring the calculated simultaneity of each route as close as possible to the target curve, and then verifies the result by an exact evaluation over the entire year.

As a result the calculation provides:

• per consumer an optimized time shift in hours (positive = delay, negative = advance of the profile),
• per route the achieved simultaneity factor.

Only routes with at least three downstream consumers enter the optimization; routes with fewer consumers keep their computed value. Consumers that do not influence any such route receive a time shift of 0.

Part-load fraction

The part-load fraction is an independent quantity per consumer in the range 0.0 - 1.0. It defines the share of the heating power that the consumer actually draws: 1.0 corresponds to the full design load, smaller values reduce the applied load accordingly. It is suitable, for example, for existing buildings whose actually drawn load is permanently below the installed connection load.

The part-load fraction scales the consumer load in the steady-state calculation and in the dynamic simulation. It is deliberately not factored into pipe sizing - pipes are sized for the full design load (reduced only by simultaneity).

Effective simultaneity

For display purposes only, the achieved simultaneity of a route can be multiplied by the load-weighted mean part-load fraction of its consumers. What is then displayed is the effective simultaneity, which combines time shift and reduced consumer load:

$$g_\text{eff} = g_\text{reached} \cdot \overline{\text{part load ratio}}$$

This is purely a display quantity in the tab Simultaneity and Part Load; it is not stored and does not enter the sizing.

Effect on the calculations

The simultaneity factor and the time shifts take effect only if Take simultaneity into account is activated; otherwise the simultaneity factor is 1.0 and no time shifts are applied. The part-load fraction is applied independently of this (even when simultaneity is deactivated).

Quantity	Pipe sizing	Steady-state calculation	Dynamic simulation
Simultaneity factor per route	× nominal power	– (realized via time shift)	– (realized via time shift)
Time shift per consumer	–	applied to the demand profile	applied to the demand profile
Part-load fraction per consumer	not applied	× applied load	× applied demand profile

• Pipe sizing: the nominal power of each route is multiplied by its achieved simultaneity factor, see Calculation method. If simultaneity is activated and no up-to-date results exist, Pipe sizing runs the calculation automatically beforehand.
• Steady-state calculation and dynamic simulation: instead of a blanket factor, the optimized time shifts are applied to the demand profiles - simultaneity thereby arises directly from the offset loads. In addition, the part-load fraction scales the respective load.
• Time-series export: when exporting the building demands, the shifted profiles can be output (option Take node time shift into account), see Editing building demands.

Results can become outdated

The calculated time shifts and simultaneity factors belong to exactly the network state for which they were calculated. If the network topology, a building demand, a custom simultaneity, the target curve or the calculation settings change, the stored results are considered outdated and must be recalculated before being applied. The tab points this out with a warning message.

Related topics

• Simultaneity and Part Load tab
• Simultaneity Calculation (dialog)
• Calculation method of pipe sizing
• Editing building demands