Why do I care about Plant Sizing?
As part of doing early stage analysis of building design, you may want to consider how different design options affect the overall central plant sizing.
For many systems envelope improvements can have a much bigger impact on air-handling unit and chiller size than on energy as a percentage of the total capacity. These improvements may lead to reductions in space requirements, equipment costs and other benefits which in turn justify the envelope strategies used.
Similarly if you're comparing HVAC options, you will want to be able to communicate to decision-makers how different HVAC system options affect the amount of plant and equipment required. A good example is the comparison between all-air systems, such as VAV systems and DOAS systems. Often the central air distribution between these systems will be substantially different.
The Plant Sizing Tab is designed to help you make these apples-to-apples comparisons, even if systems are quite different in design.
Plant Sizing - Cooling - what is it?
The top-left component in the Plant Sizing tab is the Cooling section. The aim of this is to provide you with a rolled-up comparable summary of what the total cooling equipment requirements are on a project (whatever they may be) and what they're made up of.
If you click "Expand outputs" you'll see 5 outputs and each is explained below.
All Zones Peak Coil Load
This is the total sizing for the cooling coils that are attached to any cooling equipment inside of all the zones in a building.
Typically only DOAS systems have cooling in the zone. The name is deliberately general and includes all of the following types of cooling that may exist, from the cooling requirement of a radiant floor to the coil in a fan coil unit. It's designed to help you compare the total load of different systems.
Coincidental loads vs Sum of Peaks
We use the HVAC system you've selected to determine whether we publish the peak coincidental load for all the zones or the sum of peaks.
Generally speaking, systems which use a central cooling system, like a chiller, to provide cooling are assumed to be designed to serve a coincidental load where as systems which are individual units (like PTHPs (Split Systems)) are assumed to be cumulative and we publish sum of peaks for this number.
The table below summarises which systems we use sum of peaks for and which systems we use coincidental loads for determining these values.
Zone Coil Loads - Coincidental or sum of peaks
|All air systems - eg VAV, CAV, Displacement, UFAD||These systems do not have zone cooling coils.|
|DOAS - Package Terminal AC||Sum of Each Zone's Peak Coil Load|
|DOAS - Package Terminal Heat Pump||Sum of Each Zone's Peak Coil Load|
|DOAS - Fan Coil Units with Central Plant||Coincidental Peak Coil Load for all zones|
|DOAS - Water Source Heat Pumps||Sum of Each Zone's Peak Coil Load|
|DOAS - VRF||Sum of Each Zone's Peak Coil Load|
|DOAS - Active Chilled Beams||Coincidental Peak Coil Load for all zones|
|DOAS - Passive Chilled Beams||Coincidental Peak Coil Load for all zones|
|DOAS - Radiant Floor||Coincidental Peak Coil Load for all zones|
|DOAS - Heating + Ventilation||This system does not have zone cooling coils|
Notes about EnergyPlus and what values we use
To size each zone, we look at the load that EnergyPlus tells us has been removed from the space for every hour in each sizing day. Note we take this value rather than the equipment size value because after having done comparative studies and tests, we feel this is a much more reliable output for equipment size. This means that if you take equipment sizes directly from EnergyPlus runs, the outputs may not match. You need to take the output that is the load removed from the space in that particular hour.
For DOAS systems, EnergyPlus has only recently supported supplying outside air direct to the zone units. We currently do not assume air is supplied directly to the zone cooling unit, we assume it is supplied to the zone separately. For now be aware that if you do not pre-condition the outside air from the AHU, you may be undersizing your cooling coil in the zone unit and resulting in high humidity conditions in the space.
We are working on providing an option here in the future for users to make a choice.
AHU Peak Coil Load
This is the peak load seen by the cooling coil in the Air Handling Unit for the HVAC system. This air at the pre-AHU Cooling Coil design condition being cooled down to whatever the supply air condition is.
For all systems you can adjust the design supply air dry bulb condition from the AHU using the air-icon after the AHU. For DOAS systems you can also adjust the design supply air dew-point.
All systems have an AHU of some description (even if you can set it up to behave more like a heat recovery ventilator) and for most systems cooling is on as a default.
This load often occurs on the enthalpy design day. This means that quite often the peak cooling coil load in the central AHU happens at a different time to the peak cooling for the zones.
Total Peak Coil Load
For systems with central chilled water plant, this is the coincidental peak between the coil loads occurring in the zones and the coil load seen at the AHU.
For systems with separate plant for the AHU (eg DX cooling at the AHU for a DOAS system) this is the sum of the AHU Peak Coil Load and the All-Zones Peak Coil Load.
This is the load seen by the coil(s) and does not include any system losses.
Cooling Equipment Design Capacity
This generic term is meant to describe the total capacity of cooling equipment that is needed for the building.
Some customers ask why it's not called Chilled Capacity. It is not called Chiller Design Capacity because if you were comparing a rooftop-package VAV with a VAV with central plant only one system technically has a chiller. Both systems have cooling equipment which is why we use this term.
For systems with chilled water, it will be higher than the Total Peak Coil Load if the Peak Distribution Efficiency in the Water-side Input Tab is less than 1 (the default is 0.9 - see below). This value is meant to represent losses at the design condition from heat gain in the chilled water distribution.
For systems that have distributed cooling, no losses are assumed and this value will be equal to the Total Peak Coil Load.
Chilled Water Pumps
If your system has a chilled water loop then we will calculate the total pumping capacity required.
This is a function of the Cooling Equipment Design Capacity and the delta-T entered in the Chilled Water Loop Supply and Return temperatures.