Load management

Load management or demand management is a method to adjust the electrical demands rather than the output of the plant. This can be done by for example forced switch-off of large power consumers such as sauna stoves and ovens or by limited simultaneous use of electrical appliances (Alanne, Micro- Cogeneration-I: Introduction). Today, demand management usually concerns the demand for electricity, but in the future demand management for other utilities such as natural gas or water might be possible. The main principle with the demand management controls is to shut off or reduce the power to non-essential loads. This is done in order to reduce the overall building demand, which will be beneficial for the CHP device as it can cover a larger part of the building’s demand, and thus reduce the amount of imports from the electricity grid. Typical controls are:

 Shut off or dim electric lights, equipment, or HVAC systems

 Reset the thermostatic set points on HVAC systems (if electrical)

 Reduce the load of a set of similar components by rotating one or more components “off” for a short time interval

 Turn on generators to meet some or all the building’s demand (EnergyPlus- US Department of Energy, 2013)

In EnergyPlus, the demand limiting controls implemented are intended to model some of the more common demand limiting strategies. One of the objects to use is called DemandManagerAssignmentList, and is a high level control that makes demand limiting decisions based on a list of possible demand limiting strategies. Each of the demand limiting strategy will be described in a separate object called DemandManager. Each DemandManager object will control a group of similar load objects of same type, such as lights, electrical equipment or thermostats (EnergyPlus- US Department of Energy, 2013). The Demand Manager is built into the overall solution method for the program. Three major segments of code are executed by the program for each time step:

- Exterior energy use

- Zone heat balance (surface heat balances, internal gains and air flows). - HVAC system simulation (air and plant loops).

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The exterior energy use is independent of the zone heat balance and the HVAC system simulation.

This energy use handles energy use regarding exterior lights and exterior equipment that are placed outside the building, and which do not contribute to the zone’s heat balance. In the building model used in this thesis, there is no exterior energy use as there are no exterior lights or equipment. The zone

heat balance includes all of the surface heat balances, internal heat gains and airflows. The HVAC system simulation includes air and plant loops with their associated HVAC components. The HVAC

system’s behavior depends on the results of the building’s heat balance at each timestep. During the simulation, the DemandManager is called after each HVAC simulation step. First, the DemandManagerAssignmentList object is evaluated and the DemandManager then decides if demand limiting is required depending on the current load of the building. If it is required, the demand limiting objects are limited after priority. Based on the Demand Manager Priority selected, the Demand Manager then decides which DemandManager objects should be activated. In turn, the activated DemandManager objects limits the respective load objects. As one or more of the DemandManager objects has been activated, a time step has to be re-simulated as the load conditions have changed (US Department of Energy, 2013).

There are two options for the DemandManager Priority; sequential Priority or all priority. For

sequential priority, each DemandManager in the list is sequentially activated from first to last until the

demand is reduced below the limit or until all managers are activated. A DemandManger is skipped if it is not possible to reduce the demand. Reasons that make it impossible to reduce the demand are that there is not enough load to limit, the demand is not available because of is on-schedule defined, or it is already activated because it reached its load limit during a previous time step. For All Priority, all DemandManagers in the list are activated simultaneously to achieve the maximum demand reduction. Using this priority option, only one final re-simulation is required after all DemandManagers are activated (US Department of Energy, 2013). Sequential priority is chosen for the load management applied in this thesis as some loads of the building are considered less essential than others.

The first approach in the load management will be to reduce the electricity demand below the standard values for yearly electricity in NS 3031:2007+A1:2011 (NS 3031:2007+A1:2011, 2007/2011). This value is in total 28.9 kWh/m2 a. The building is implemented with relatively low energy demand for lighting, while the energy demand for electrical appliances are high. Therefore, the majority of the demand management should be done here.

The load management is implemented such that the CHP generator can be able to cover the whole electricity demand. The maximum electricity demand limit is therefore set to be 4000 W. This is implemented with a safety fraction of 0.8, which implies that electricity demand over 3200W is adjusted by a damand manager assignment list as was explained in the previous paragraphs. This is under 5500 W, which is the amount of electricity possible to produce by the generator. Avoiding the high peaks will make it possible to reduce the dependence on the electricity grid as the CHP unit can produce a larger amount of the buildings demand, especially since the thermal and electrical peaks often do not happen at the same time. Load management will be tried implemented in both heat-led and electricity-led operation to see where the effect of the load management will have greatest impact on the operation of the CHP. When implemented in electricity-led operation, the demand limit is set to be 5500 W, instead of 4000 W since it is expected that the generator is more capable at meeting peaks when it follows the buildings electrical demand. However, since high electricity production is followed by high heat production, it is expected that high electricity peaks may result in overheating of storage tank.

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