Primary system (Plant)
The plant can be modified by selecting components from the side bar, double clicking on them or dragging them into the plant.
When choosing “Build plant model” in the upper right corner of the plant form, you will get a schematic view of all the components of the plant at advanced level, see figure below. The main components are the boiler and the chiller. The domestic hot water settings are given under Extra energy and losses in the General (Building) tab but may be altered here.
The default plant
The default model (see figures above) consists of the Standard boiler and chiller (selectable under Combined systems) and the Generic heating and cooling system (selectable under Distribution systems). Double-clicking on the items will open forms for the settings. By default, the energy carrier is fuel for the boiler and electricity for the chiller. The generator efficiencies for the energy carriers of the Standard plant are given in the General (Building) tab under “Defaults”.
The hot water supply temperature is by default 60 °C for the AHU, while for the zones it is determined from the outdoor air temperature by a piecewise proportional controller. The boiler pumps are by default controlled to run when the outdoor temperature exceeds 18 °C. This value can be altered in the Distribution systems form. The chiller has two cold water temperature settings, one for the AHU (5 °C) and one for the zones (14°C). The chiller pump may be controlled by a schedule and is by default always on.
The standard boiler
The IDA ICE standard boiler converts purchased energy, e.g. gas, electricity or district heat, to warm water with given temperature and pressure for circulation through water-based heat exchangers in the building. It also uses energy for production of domestic hot water and pumping. The boiler efficiency is by default constant, as specified in the Defaults form. Open the component to view key parameters.
It is also possible in the standard boiler to model water heating efficiency as a function of boiler temperature and part load. The model and parameter definitions used are the same as those of EnergyPlus. There is database support for this performance data.
Pumping power consumption for heating water circulation can be specified in three ways: (1) proportional to the water flow through the boiler (default), (2) as a proportion of distributed heat, or (3) as a polynomial function of the water flow. The third option follows the conventions of ASHRAE 90.1
The first option assumes an ideal pressure-controlled pump with constant efficiency. Alternatively, by setting the efficiency to some large number and specifying the k1 parameter, pumping effort can be given in proportion to distributed heat. (PSetMax should still have a somewhat realistic value.)
In the third option, the efficiency of different types of pumping solutions and control are reflected by a user provided polynomial function. This requires the input of a design mass-flow as a point of reference. Two standard curves from ASHRAE 90.1 are provided.
Pumping power for domestic hot water circulation can also be specified via k2, which specifies pumping power as a fraction of domestic hot water heating power.
The setpoint for the hot water supply temperature comes from a special controller component connected to the boiler. The controller provides a graph showing the setpoint as a function of the outdoor air temperature. (Press F1 in the dialog for detailed instructions.)
The standard chiller
The IDA ICE standard chiller and its circulation circuit operate in a similar way but differ in some aspects from the boiler. The chiller uses by default electrical power to produce chilled water at two different constant temperatures (but with the same pressure). The colder water, by default 5 °C, supplies the AHU. The somewhat warmer temperature, by default 14 °C, goes to the zones.
Similarly, as for the boiler, EnergyPlus correlations for modeling temperature and part load behavior may be applied. Pumping power is also specified as in the boiler.
The distribution system
By dragging Advanced heating/cooling system from the palette to Distribution systems the user is able to define multiple water circuits to the AHUs and the room units in the zones. It is also possible to add a Night setback of the supply temperature in each circuit through a schedule. More information is given here.
System parameters
Default values for components are given from the system parameters. More parameters are given in the Outline tab.
Design of dT for free heat/cold sources
The temperature rise/drop for a brine circuit extracting heat/cold from a free source. Used for computing the design mass flow.
Pump efficiency for free heat/cold sources
The efficiency of a pump for a brine circuit getting heat/cold from a free source.
Design temperature difference over heat exchangers in tanks
Affects the UA-value of internal heat exchangers.
Design pressure drop of brine circuit
Used for calculation of the pump power.
Fluid type in brine circuit
Choice of brine fluid. Solar thermal has its own brine circuit, set in the Solar thermal form.
Freezing point of brine
Give freezing point for the used brine fluid.
The ESBO plant
By making changes in the default Plant, more complex plant models may easily be constructed. In the General tab, many different system configurations can be described by dragging in various components from the palette. See the ESBO documentation for more details about available components.
In the next few sections, we describe the principles for the configuration and control of the default ESBO systems. Here, the narrative will rely on system schematic diagrams as displayed in the Advanced level. Press Build plant model in order to generate the Advanced level for the ESBO Plant.
Purpose and ambition of the default generated systems
In economically feasible real systems, many compromises must be made. The total number of valves, pumps and connections must be reasonable and control logic must not be overly complex. In contrast, the purpose of the ESBO default systems is to exploit all significant energy flows based on a minimum of input data from the user even if the physical components needed are not economically justifiable in a real system. The generated systems are technically realistic but might, at least for smaller installations, contain too many parts to be economically attractive for realization in real life.
Furthermore, the control systems employed are extraordinarily “decentralized.” In order to avoid the combinatorial explosion of having to define a central control system for each possible system configuration, each subsystem is maximally self-sufficient. A flow controller for, e.g., a ground source circuit will simply check when it is possible to deliver heat (or cold) and does this until it is not energy-wise economical to continue (when pumping costs will exceed the benefits). Long-term control strategies, for example to try to seasonally balance a large thermal store, is out of scope of the automatically generated default systems. Whenever the short-term benefit of a certain flow is in place, the flow will be activated.
The Generic tank models
All generated systems are centered around two stratified water tanks, one for hot and one for cold water. These two tanks are mandatory for most system combinations, also for situations where a real system would not include a tank. Furthermore, the tanks are by default equipped with valves that will allow each entering and exiting flow to be made at the optimal height in the tank. This way, minimal buoyant mixing will occur. Real tanks exist that behave in a similar fashion, using for example heat lance technology. The tank is also by default equipped with a bypass (shunt) circuit for each client-side (discharge) connection. This way, return water from the same circuit is used first to attain the target temperature of the required supply flow, thus minimizing the flow through the tank.
For situations where a storage tank is not part of the concept to be investigated, the volume of the mandatory tanks is set to be fairly small with respect to surrounding systems, but not to zero. The tanks can in this situation be thought of as “temperature switchboards” that minimize the amount of temperature mixing in the system. Tank volume would then represent the thermal mass of the actual HVAC system.
The default height to diameter ratio (shape factor) of the tanks is set to 5, representing a fairly tall tank with, correspondingly, only a small amount of conduction between the water layers. In order to create a less ideal tank and ultimately approaching a completely mixed tank, the shape factor can be set smaller, so that conduction will even out temperature differences between layers. Another way of gradually approaching a well-mixed tank is to decrease the number of layers.
At advanced level it is possible to switch to a “non-ideal” tank in which you instead select fixed vertical positions for tank ports. Buoyant mixing will then occur if a hot water stream enters the lower part of a tank. In all other senses it does not differ from the default “ideal” tank model. Discharge-side bypass flow is controlled by the SHUNT parameter.
For a full mathematical description of the tank model, see the Forum post. Open the Forum from IDA ICE before following the link.
An example of a generated system
The figure below shows an example of how the General tab may be populated with models. Some model has been chosen for each of the possible functions. Brine to water vapor compression machines have been defined for base heating as well as for cooling. Both ground heat exchange and ambient heat exchange have been defined. Below is a relatively detailed account of the workings of the plant model for this case, where we will go through and explain each major subsystem below.
Organized around the two water tanks, the top one for heat and bottom one for cold storage, are the various circuits that draw or feed tempered water from and to the tanks. Let us start with the client circuits to the right, starting from the top right corner with the DHW circuit.
Domestic hot water (set under “Extra energy and losses” in the General (Building) tab) is drawn from the hot tank, by the leftmost PMT-object, which has a mass flow signal into it that gives the required DHW mass flow. The make-up water from the water mains is furnished by the rightmost PMT-object and the given inlet temperature. Each client circuit, including this one, feeds a temperature setpoint into the tank. In this case this is the temperature setpoint for the DHW (55⁰C). Note: This temperature setpoint will be used also when there is no actual flow of DHW, e.g., a heat pump controller will always try to keep this temperature level available in the tank (at the expense of better COP). Set a lower DHW temperature to avoid this effect if it is undesirable.
As a simplification, no separate “tank-in-tank” has been defined for the DHW in the default configuration. The heat exchange between DHW and surrounding water is regarded to be infinite and immediate. A further simplification is that losses from a DHW circulation circuit are not reflected.
The meter objects to the top right are used to record results about DHW production.
Next below is the AHU hot water circuit, which is connected via a pump to the air handling unit, providing it with heated water at given pressure and temperature. Similarly as for the DHW, a temperature setpoint (60⁰C in the example) informs the tank about the temperature that is required by this client. Also in the AHU box is an on-off controller for turning off the hot water circulation to both AHU and zones when the ambient temperature is higher than, here, 18 degrees. Actually a 3 ⁰C dead band is applied to this switch (only visible if the component is opened) to avoid frequent switching events.
The Zone hot water circuit is similar but has a more elaborate arrangement for the calculation of the temperature setpoint, allowing it to be a function of ambient temperature as well as a of a night set back schedule.
The AHU cold water and Zone cold water circuits connected to the cold tank are found in the lower right corner. They have in this example simple fixed setpoints, 5 ⁰C and 14 ⁰C, respectively, throughout the year.
On the production side, at the top on the left side, the Solar thermal collector model is located with a circulation pump and a separate expansion vessel. This separate brine circuit feeds into a heat exchanger that is located at the bottom of the hot tank. In-tank heat exchangers are also served by idealized heat lance technology. In the default set-up, the solar collector will never prioritize the production of high-temperature DHW, but a fixed (in the example 5 ⁰C) temperature difference is upheld in the circuit by a PI controller connected to the pump, instead maximizing the collected amount of heat. When there is no sun, the pump will not operate because the temperature difference over the collector will be negative or below 5 ⁰C even at the minimal circuit flow which is always maintained.
A separate monitoring control circuit at the bottom of the Solar thermal area keeps track of collected heat over a longer period of time (days). This signal is used to determine how much the heat pump in the Base heating circuit should be utilized. During the winter, when there is little hope for sun, the heat pump will always attempt to keep the tank as full as possible, to avoid using the Top heating.
Next to the right is the Top heating circuit, which is the backup for keeping the top of the tank at the maximal required client temperature. It feeds directly into the tank water, and since expansion vessels are included in the tanks, only a pump is required in this circuit. The control circuit measures the top level water temperature and asks the top up heater to heat, provided the Base heating is already fully engaged.
Next below is the Base heating circuit, where the condenser circuit of the brine to water heat pump feeds into the tank directly. The heat pump and the condenser circuit pump are controlled by a PI controller (baseCtrl) which attempts to keep the fill ratio of the tank at a given setpoint (which is computed by the Solar thermal monitoring circuit.) The fill ratio is defined as the degree at which the tank is filled with water at the highest required setpoint, i.e. if all water is heated to the highest setpoint, the fill ratio is 1, while if the whole tank holds the ambient temperature (20 ⁰C), the fill ratio is zero.
Furthermore, the base heating heat pumps will have a speed limitation of the condenser pump circuit that will be active when the heat pump should prioritize hot water production, i.e. the pump speed is limited in order to force the condensing temperature to increase.
On the evaporator side, the brine to water heat pump is connected to the Brine circuit. The brine circuit is a more unconventional design and we will explain how it works here. The basic idea is that all free heat and cold sources are connected in parallel to a single brine circuit. The circuit will alternate to feed units that require heat (such as the present heat pump) and cold, i.e. if free sources are available at the same time for both heating and cooling and both a heating and a cooling need exists, a choice has to be made as to which one of these will be satisfied. The idea behind the design is that this collision of interests occurs only rarely.
To illustrate the function of the brine circuit, let us look at what happens with the present base heating evaporator brine flow. Once the base heating heat pump starts to operate, the temperature of the return side brine will drop after the condenser. First the brine passes through a “PMT tap” component, which also receives a signal to open the circuit through the evaporator as the heat pump starts. Should it be beneficial to run the cooled water through a heat exchanger at the top of the cool tank, this is done. After returning back through the PMT tap, the brine is returned to the return brine manifold in the Brine box.
In the event that no free supply circuits exist or are able to operate, and the evaporator was able to discharge to the cold tank, the flow in the brine circuit will be upheld by the pump in the brine box, which monitors both PMT tap components for possible beneficial flows. More commonly, one or several of the free supply circuits will instead pick up the heating need of the heat pump evaporator. Let us look at one of these, the ambient heat exchanger.
The current Ambient HX circuit, found immediately to the left of the brine box, is in the example a fan assisted ambient heat exchanger, which is able to both cool and heat the brine when needed. All of the free supply circuits rely on a special pump/controller object “FreeSupCtr” that monitors the temperature difference between the supply and return brine manifolds. When the temperature of the return manifold drops, it is a signal that heating is required, and if the free source circuit is able to meet such a need, it will start its flow (and in the example, the fan of the ambient hx). The circuit will, in heating mode, be operated at a degree which keeps the contributed flow above (by default 5 ⁰C) the brine return. The operation will be continued until it is no longer beneficial, for example because the fan power is getting close to the useful delivered power.
All Free supply circuits operate independently of each other in trying to satisfy the current predominant need. Any circuit which can help rise the supply side brine temperature will be operated in a heating situation.
The free supply circuits can also be operated without any condenser or evaporator in the loop. Suppose for example that the bottom of the hot tank has a temperature which is well below outside air temperature. In this situation, the PMT tap component will open its circuit and the ambient heat exchange circuit will start charging the tank directly. This type of situation is of course much more common when it comes to cooling; the free sources will then often be able to directly feed into the cold tank.
In the example, the Cooling brine to water chiller operates in a similar way. Any useful heat after the condenser is directly tapped into the hot tank, a free supply circuit will pick up the cooling need and start to operate.
There is nothing that formally prevents both the heat pump and the liquid chiller in the example to operate in parallel, both cooling and heating the brine circuit simultaneously. In this situation, the brine pump will initially keep the flow in the circuit going but after some time one of the two compression cycles will “win” and create a net cooling (or heating) need, which then is likely to be fulfilled by a free supply circuit.
The present example, with two brine connected vapor compression machines and multiple free supply circuits represent the most complex type of system that can be described. In all other situations, some of these components or connections are missing, creating simpler system schemata with a lower number of operation modes.
List of objects available at the side bar:
Generic solar thermal Solar thermal collector - location visualized in the 3D view
Ambient air to water heat pump Variable speed heat pump based on industry std compressor polynomials
Ambient air to water heat pump (simplified) Simplified heat pump model (variable speed not possible)
Brine to water heat pump Variable speed heat pump based on industry std compressor polynomials
Brine to water heat pump (simplified) Simplified heat pump model (variable speed not possible)
Generic combined heating and power base heater
DX Exhaust air Heat pump Must be combined with the AHU "Exhaust air for DX heat recovery"
Brine to water chiller Variable speed chiller based on industry std compressor polynomials
Brine to water chiller (simplified) Simplified chiller model (variable speed not possible)
Heat exchange with given temperature source
Dry or wet air to brine heat exchanger
Liquid circuit for AHU heat recovery Must be combined with the AHU "Exhaust air with liquid heat recovery"
Advanced heating system Allowing for multiple heating distribution systems
Advanced cooling system Allowing for multiple cooling distribution systems
IDA ICE standard boiler Standard combined heating/storage system (unlimited)
IDA ICE standard chiller Standard combined cooling/storage system (unlimited)