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I am working on an Energy Management problem. The objective is to minimize the electricity bill for the customer.

I have a time-series data with 15 min. intervals spanning the course of 1 year. The main controllable system is a hot water boiler which can be switched $ON$ or $OFF$, $1$ or $0$ respectively.

These are the requirements:

  • The boiler can only be switched (either $ON \rightarrow OFF$ or $OFF \rightarrow ON$) max. $n$ times a "day"
  • The "day" is not necessarily considered from 12:00AM to 12:00AM but it can be from 06:00AM to 06:00AM or any other 24 hour duration, it depends on the customer

I understand that there is a way to generate this constraint for 1 day using:
$\sum_{t=0}^{95} abs(b_{t+1} - b_t) \leq n$
where, $b_t$: boiler status at time $t$

But this uses $abs()$ which makes the problem non-linear. The entire problem, constraints and objective function are linear and hence, this constraint must also be linear. Any suggestions would be appreciated.

I am using Pyomo for optimization and the data is present as a Pandas Dataframe.

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1 Answer 1

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The abs function is piecewise linear, so you can represent it with two linear inequalities as shown in https://docs.mosek.com/modeling-cookbook/linear.html#absolute-value.

In your case, $\sum_{t=0}^{95} abs(b_{t+1} - b_t) \leq n$ can be formulated as $$\sum_{t=0}^{95} u_t \leq n,\quad u_t \geq abs(b_{t+1} - b_t).$$ and thus linearized as $$\sum_{t=0}^{95} u_t \leq n,\quad -u_t \leq b_{t+1} - b_t \leq u_t.$$

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  • $\begingroup$ what does $\Delta b_t$ represent? is $\Delta b_t = b_{t+1} - b_t$? $\endgroup$ Commented May 23 at 8:40
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    $\begingroup$ I updated my answer renaming $\Delta b_t$ to $u_t$ to avoid confusion. $\endgroup$ Commented May 23 at 9:24

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