Years, periods, and time slices
This page describes how time-related concepts are represented in the MESSAGEix framework and stored in ix modeling platform.
Years and periods
year set—in a
message_ix.Scenario and in the MESSAGEix model formulation—is used to index parameter dimensions with the names “year”, “year_act”, “year_vtg”, etc.
Elements in this set are representative years within a period of time.
MESSAGEix treats periods as contiguous, but
year elements need not be consecutive.
The representative year is always the final year of the corresponding period.
- Example 1
In a Scenario with consecutive
yearelements [1984, 1985, 1986, …], the element
1985refers to the period from 1985-01-01 to 1985-12-31, inclusive.
- Example 2
In a Scenario with non-consecutive
yearelements [1000, 1010, 1020, …]:
1000refers to a period that ends 1000-12-31, i.e. at the end of the calendar year 1000.
1010refers to a period that ends 1010-12-31, i.e. at the end of the calendar year 1010.
1010therefore refers to the period from 1001-01-01 to 1010-12-31, inclusive.
duration_period stores the duration of each period, measured in years, indexed by elements from the
MESSAGEix only supports
year set elements that can be represented as integers, and therefore periods with durations that are a whole number of years.
Other parameters may be aligned to the start or end of periods.
- Example 3
A Scenario has
yearelements [1000, 1010, 1020, 1030, …], and a technology parameterized with
technical_lifetimevalue of 20 years for the key
Capacity associated with this technology is constructed at the beginning of the period denoted by
1010, i.e. as of 1001-01-01.
By the end of this period, 1010-12-31, this technology has operated for 10 years.
The next period, labelled
1020, ends in 1020-12-31. By this date, the technology has operated for 20 years, equal to its technical lifetime.
The following period, labelled
1030, begins on 1021-01-01. The capacity created in
year_vtg=1010is beyond its technical lifetime, and unavailable in this period.
time set is used to index parameter dimensions with the names “time”, “time_origin”, “time_dest”, etc.
These are variously referred to as “(sub-annual) time slices”, “time steps”, or other names.
Elements in this set are labels for portions of a single year.
The special value
'year' represents the entire year.
year element refers to the representative, final year within a period, using
time together denotes a portion of that specific year.
- Example 4
In a Scenario with
yearelements [2000, 2002, 2004] and
timeelements [summer, winter]:
2002refers to the period from 2001-01-01 to 2002-12-31 inclusive, which has a
duration_periodof 2 years.
timeelement ‘summer’, used alone, refers to a portion of any year.
In a MESSAGEix parameter indexed by (
time, …), values with the key (
2002, ‘summer’, …) refer to the ‘summer’ portion of the final year (2002-01-01 to 2002-12-31) within the entire period (2001-01-01 to 2002-12-31) denoted by
Duration of sub-annual time slices
The duration of each sub-annual time slice should be defined relative to the whole year, with a value between 0 and 1, using parameter
For example, in a model with four seasons with the same length,
duration_time of each season will be 0.25.
Please note that the duration of time slices does not need to be equal to each other.
This information is needed to calculate capacity of a technology that is active in different time slices.
Time slices can be represented at different temporal levels, using sets
This helps introducing a flexible temporal resolution, e.g., by representing some technologies at finer time resolution while others at
When there are more than one temporal levels, e.g., “year”, “season”, “month”, “day”, etc.,
duration_time is defined for time slices at each temporal level separately.
The sum of
duration_time of time slices at each temporal level must be equal to 1.
For example, in a model with 4 time slices as “season” and 10 time slices as “day” under each “season”,
duration_time of each “season” and “day” can be specified as 0.25 and 0.025, respectively.
By default, the unit of
ACT is treated per year in the GAMS formulation for different time slices. This means values reported
in time slice “year” and “month” both have the same unit (e.g., GWa). However, the user can report the values across parameters
and variables with different units relative to the length of the full year. For example, the user can report
ACT in units of
“GWa” and “GWh” for time slices of “year” and “hour”, respectively, in the same model. To activate this feature, the parent time slice
for which the relative units are desired should be specified by set
time_relative. This will ensure that parameter
is effective. Otherwise, this parameter is filled by value of 1, meaning that the units will be treated uniformly across
different sub-annual time slices.
interest_rate in MESSAGEix is defined for a period of one year, therefore, for periods of more than a year, the discounting is performed in a cumulative manner.
- Example 5
Using the same setup as Example 2:
Discounting for the element
1010involves discounting for years
1002, … ,
Using the standard PV formula, we have that, for the year
1001the discount factor would be \((1 + interest_rate)^(1000 - 1001)\), for the year
1002the discount factor would be \((1 + interest_rate)^(1000 - 1002)\), and so on.
Therefore, the period discount factor for the element
1010is \(df_1010 = (1 + interest_rate)^(1000 - 1001) + (1 + interest_rate)^(1000 - 1002) + ... + (1 + interest_rate)^(1000 - 1010)\)
Analogously, the period discount factor for the element
1020is \(df_1020 = (1 + interest_rate)^(1000 - 1011) + (1 + interest_rate)^(1000 - 1012) + ... + (1 + interest_rate)^(1000 - 1020)\)
So, if we have a cost of
K_1010for the element
1010, its discounted value would be
df_1010 * K_1010, which means, all the years in element
1010have a representative cost of
K_1010that is discounted up to the initial
yearof the setup, namely, the year
In practice, since the representative year of a period is always its final year, the actual calculation of the period discount factor within the model is performed backwards, i.e., starting from the final year of the period until the initial year.