A full operational strategy for an EB and HP, including all the relevant markets analyzed in the previous sections, would result in multiple interlinked decisions to be taken at dif-ferent times. As was realized from the previous sections, difdif-ferent markets have difdif-ferent structure and time horizons, which makes such operational strategy highly complex. Figure 3.2 displays a time line of decisions to be taken, including both, the ordinary heat market, the Elspot market, the one day-ahead reserve market, the regulating market and finally the real-time changes based on the realized heat demand. In the vertical axis additional information on the decision is outlined. ”Basis” highlights the information available at the time of the decision and other previous constraining decisions. ”Characteristics” indicates the complexity of the given market/decision and the relative importance of an operational strategy.
scheduling FNR offering Regulating power Nord Pool spot
Figure 3.2– Outline for an operational stategy for an EB and HP including both heat and electricity markets. Markets are ordered according to the time at which decisions are made.
As this system is both complex and interlinked, all decisions should optimally be included into one model describing the full system. This could act as a decision support tool in all the decision phases outlined in Figure 3.2. However, such a model requires extensive modelling and might be computationally intractable, due to the complexity of the system and the excessive amount of data that would serve as an input. Instead, the most
impor-3.5 Chapter summary 33
tant decision(s) must be identified and used as a starting point for the development of an operational strategy.
Even though the heat dispatch, see column 1 in Figure 3.2, is very important, it does not provide information on how to operate the HP and EB, but merely how much these optimally should produce. Naturally, this is very important if the aim is to ensure the maximum dispatch, but less significant here, where the goal is an operational strategy.
The FNR market, outlined in column 3, is very small and competition has decreased the profitability from this market. The high uncertainty in this market, described in Section 2.3.2, makes the risk of uneconomical situations higher if a specific low-risk strategy is not developed. In addition, the current heat dispatch procedure does not allow for bids to be affected by this market.
The regulating market, summarized in column 4, is an interesting market to participate in.
A CHP unit could benefit from this market, as power production might have to be adjusted to meet the realized heat demand. The EB and HP could be included by offering flexibility during period they are not competitive in the heat market. However, if a system comprising both CHP, HP and EB units is available, the flexibility of the HP and EB could make this market less important.
Essentially, the development of an operational strategy for an HP and EB starts by defining how to integrate these units into the ordinary heat and power scheduling that occurs before the spot prices are known, but after the heat dispatch has taken place, see column 2. The other markets and services are secondary options to increase the profit, while the first step is to model the integration of a HP and EB in the heat and power production planning in a CHP system.
3.5 Chapter summary
This chapter presented a framework for developing an operational strategy for a HP and EB. The potential of relevant markets were analyzed, and a time line for an operational strategy comprising all relevant markets was presented. The challenge and importance of each decision was stressed, and it was concluded that the main challenge is to find the operational strategy, which includes the HP and EB in the ordinary heat and power scheduling and subsequently offer power to the Nordic spot market.
In order to make a heat and power schedule for a CHP system, a mathematical model seems necessary as the complexity of the system and decision process makes it difficult to choose optimally. This will be the scope of following chapters.
Chapter 4
Operation models for a CHP system
Based on the analysis of the framework carried out in the previous chapter, this chapter introduces a model describing the heat and power operation in a CHP system including a HP and EB supplying a district heating network. The CHP system comprise two CHP units, a HP, an EB, a small local and a large heat accumulator (storage), s1 and s, respectively.
The small local storage is only connected to the HP. Electricity from the CHP units is used to supply the EB directly. The remaining electricity is sold on the Nordic day-ahead Elspot market. This system, without the HP and EB resembles a realistic integrated heat and power production system in Copenhagen. The EB and HP constitute two additional units to increase flexibility and take advantage of low electricity prices.
4.1 System framework
Figure 4.1 shows a simple overview of the system. Red dashed lines represent heat transfer from normal production; black arrows represent electricity inputs or outputs. The two CHP units, one extraction unit and one back-pressure unit, produce directly to the transmission network and to a large heat accumulator.
The transmission network supplies several local distribution networks of which only two are shown here. The HP is, due to temperature limitations, only connected to a local distribution network and a small local heat accumulator. The HP operates as a negative load for the total system, meaning that it is assumed that the size and heat demand of the distribution network is large enough for the HP to produce at any time. Both the HP and EB consume electricity and offer heat.
35
Heat accumulator
Transmission grid CHP
CHP
Heat pump
Electric boiler
Local distribution grid
Local distribution grid Heat
accumul ator
Extraction unit Backpressure unit
Figure 4.1– Overview of a district heating system comprising two CHP units, an EB, a HP, a large and a local heat accumulator.