#901 Use of 'net' tag for thermal energy

Jaap Balvers Wed 31 Mar

The current definition of net is in combination with elec energy exchange with the grid. We also see use cases for a {net thermal energy} sensor or equip that registers the net amount of thermal energy.

This is useful in a system that can change direction of energy delivery, e.g. heating in summer and cooling in winter. In our case, the example is an ATES system, but I can imagine drycoolers and other systems that also change the direction of energy delivery depending on conditions.

I would like to broaden the net definition to also cover these situations.

Any thoughts you have are very welcome.

Brian Frank Wed 31 Mar

The definition of the net tag itself is already pretty generic: "Difference between import and export".

Do you have proposed language that would make it even more generic?

I think the key point is that it should be clear which direction is negative vs positive. In the case of energy Haystack standardizes on the "load convention":

net = import - export

Jaap Balvers Wed 31 Mar

Yes that's true, though thermal energy does not have to interact with a grid and import/export are not the most fitting terms in my opinion. Maybe:

The difference between import and export or between heating and cooling.

For heating and cooling I think it should be:

net = heating - cooling

Brian Frank Mon 19 Apr

I talked with Stephen Frank about this today, and I do not believe it is so simple. In general the idea of net in fluid energy transfers is the difference between production versus consumption. But consider a chilled water system, in this case the positive value would be energy supplied thru the chilled water (even though the heat transfer is the opposite way). Or in the case of a heat pump, the idea of positive values vs negative values might flip based on whether you consider the unit in a cooling mode vs heating mode. I think even in your original post you are implying that positive values are based on knowing whether the system is in heating or cooling mode?

In terms of importing energy vs exporting energy (be it thermal energy or electric energy), I actually think the existing definition of net is correct isn't it?

Jaap Balvers Mon 19 Apr

Hope I'm not overthinking this... I think if you stay with the definition of importing vs exporting energy, the net definition can be used as-is for thermal energy if it is a meter for either cooling or heating and the cool or heat tag (I think?) is used to define the positive values i.e. what the direction of energy consumption is meant to be.

In case of a reversable process such as a heatpump (or an ATES system where the net thermal energy balance is a parameter from the legal permit of the system), I'm not sure the definition is sufficiently clear. In the ATES case, there usually are two termal energy meter equips; one for cool and one for heat. But the value we also want to register is the net sum of these, being heating minus cooling.

I don't see this as import or export, because in my experience these words are used to describe interaction of a building with the public energy infrastructure / grid outside a building.

Are you saying the current definition of net covers this type of net thermal energy meter? If so, I'm all for not changing anything that isn't broken of course.

Stephen Frank Tue 20 Apr

Chiming in, since I spoke with Brian yesterday about this. I agree import vs. export is electric grid language, but production vs. consumption more generally is the same basic idea. Where we've run into trouble is defining the "positive" direction of heat flow (or, for thermal meters, expected delta T). Examples...

  • CHW Production: System is absorbing heat (leaving water temp < entering water temp)
  • CHW Consumption: System is supplying heat (leaving water temp > entering water temp)
  • HW Production: System is supplying heat (leaving water temp > entering water temp)
  • HW Consumption: System is absorbing heat (leaving water temp < entering water temp)

So obviously heat flow flips, but typically someone observing these meters in a system would expect a positive value for both production and consumption in the correct context (e.g. positive thermal energy produced from a boiler plant; positive thermal energy consumed by a building AHU).

For a system like a ground loop or an ambient district loop, with reversible heat exchange from heat pumps, it becomes more complicated as you note. You have to decide whether heating or cooling is "positive" as well as which direction of heat flow is "positive". A building consuming heat from the ambient loop is the same as a building producing cooling for the ambient loop.

Some of this could probably be handled using production or consumption (or similar tags) to indicate expected direction of energy flow, e.g. a chilled water production energy point would be supplying chilled water (absorbing heat), and always expected to be positive (as export energy would be in an electrical context). Then the net tag could be defined with a particular sign convention (e.g. consumption - production) and only used in situations where such a reversing thermal energy flow is expected. I'm not advocating for this or any particular solution per se; more just pointing out all the considerations when trying to adapt a net tag for thermal systems so as to prompt conversation.

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