exhaust air liquid heat recovery loop

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Hi all,

I will resist the temptation to say ?happy? new year. Perhaps, for 2021, the
expression ?moderately relieved? new year is more appropriate (it?s
certainly more British). I hope as many of you as possible have avoided
covid. I know there are some members of this list who have not been so
fortunate. God speed your recovery.

In IDA ICE, there is an option to model a liquid heat recovery loop in the
exhaust of an air handling unit (image attached). In essence, this is just
putting a cooling coil in the exhaust. Heating, including heat recovery, is
after all as much a cooling process as it is a heating process. This cooling
coil can then integrate with some form of heat recovery/ reversible chiller
? a bit like a run-a-round coil heat recovery on steroids!

This works in theory. However, I was just wondering how prevalent it is in
practice. Would I be na?ve to present this as an option to clients? I know
I?ve not come across this in the UK. Is it just a Scandinavian thing?

I?d be very grateful to hear of any projects that have successfully (or
unsuccessfully!) integrated heat recovery via exhaust air liquid loops.

It seems to me that the benefits could be manifold:

* ?decoupling?: Heat recovery potential increases as the building gets
warm, but with traditional plate or thermal wheel heat recovery the demand
for it simultaneously decreases. By buffering the heat to water, or
re-directing it elsewhere (e.g. SHW) heat recovery becomes more general
purpose.
* A useful load for the chiller during periods of low chiller load.
There are well understood problems with chillers cycling on/off during
periods of low load ? aka ?low delta T syndrowm?. By dual purposing chiller
equipment for heat recovery, it provides a stable baseload for chillers
during these periods.

Kind regards

Chris

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Chris,
I don't think I understand the system....
That clearly means it's not something common for my portion of the world!
On the other hand, it sounds no different than energy recovery - can you clarify a bit, please?

? The world is having a crisis of reason. I don?t think the world is having a crisis of faith. If anything, there is plenty of faith around, in both good and bad things. In some ways, there is altogether too much faith, and too little reason.

Jim Dirkes 1631 Acacia Drive NW Grand Rapids, MI 616 450 8653

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Hi Jim,

In a generic sense, this is one of a number of different systems that capture exhaust heat via a coil in the exhaust. In the list below I am trying to establish if anybody has

Similar systems

1. We?ve already seen run-a-round coils described
2. Exhaust air heat pumps have been a very common system for residential. They tend to use the refrigerant as the heat transfer medium. Some manufacturers of these units are :

a. https://www.ivprodukt.com/products/home-concept-ecoheater
b. https://www.nibe.eu/en-eu/products/heat-pumps/exhaust-air-heat-pumps/NIBE-F750-_-237

3. Supply and extract air handling with integrated heat pump augmenting (straddling) a plate or wheel heat exchanger. They tend to use the refrigerant as the heat transfer medium. Some manufacturers of these units are :

a. https://www.genvex.com/en/products/air-ventilation---air-heat-pump/premium-preheat-500
b. https://www.youtube.com/watch?v=hzVOTjH_GIg &feature=emb_logo&ab_channel=Fl%C3%A4ktGroup

The system I?m describing

The heat transfer medium is water or brine (like the RAR coil), but this is piped to a compressor remote from the AHU. The attached is a crude mock-up of such a system using IDA?s ESBO interface (this has not been validated). Apart from a gas boiler for top-up heating, it would be an ?all-electric? system. Britain and Scandinavia both have relatively low carbon electricity now. The fact that recovered heat can be re-used in a very general purpose way at a potentially high COP is quite an attractive idea. We may even pre-heat service hot water. Note also that the sum of UA for all the heaters in the zones will be much bigger than the heater coil?s UA in the AHU itself (meaning supplied heat can be a lower grade). It may also remove some of the perceived penalties of reheat (i.e. heat pumps simultaneously make both cold and heat).

I?m searching for an appropriate quote?

There are two major products that came out of Berkeley: LSD and UNIX. We don't believe this to be a coincidence.

Cheers

Chris

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Chris, I?m sorry I didn?t quite understand your scenario before. Something like this, except it would be a heat pump to make hot and cold
Integrated Chiller - Konvekta AG

We?re primarily seeing the runaround type or plate-and-frame direct heat exchange instead of using a heat pump to boost the temperatures, although I can see some appeal.

What type of buildings are you looking at this for?

David

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Hi David,

Yes. Exactly. Dehum is also a good use-case as well, requiring simultaneous cooling and heating ? which the heat pump gives.

At the bottom of https://www.konvekta.ch/network-recovery-systems.html ?Konvekta uses the DOE-2 building simulation software as a basis to design an energy recovery system.?.

In terms of buildings, I was actually presenting this as a bit of a ?blue sky? option for some older hospital estates that we?re doing some front end strategy work for. Many of the current systems use RAR coils. However, most have no heat recovery at all and we?re considering how to retrofit it without reworking duct routes to accommodate S&E air-handlers with plate heat recovery.

I?ve seen this kind of system on laboratory jobs more recently, but not in hospitals or in a retrofit context.

Kind regards

Chris

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I don?t think I can add much to the original prompt/discussion, however just chiming in to add a voice of consensus from the gallery, if only for your collective sanity. I have seen runaround loops implemented a few times in the simplest sense (closed water loops with glycol/antifreeze capturing exhaust stream heat specifically to precondition incoming OA). Effective energy savings potential during extreme heating/cooling seasons, however the associated pump circulation energy can eat up savings potential during nice weather in between (seasons/temperature based controls advised).

These systems at sufficient scaling and in climates with extreme winter/summer preconditioning needs (in my specific experience: large laboratory / healthcare facilities with high volumes of 24/7 exhaust/makeup in the Midwest US) can definitely pencil favorably for implementation in retrofit scenarios. I have never personally encountered refrigeration loops applied as a solution to this specific sort of building exhaust heat recovery challenge, but it sounds like a great idea on paper. Whether this really pencils well however will depend in part on relative proximity of the OA inlet and exhaust airstreams. Considering the large scale applications in my portfolio of experience, I?d observe the points of exchange (OA inlet / Exhaust stacks) were typically far removed, spatially (basement/ground level AHU?s pulling in air vs rooflines/attics for exhaust) ? so very long compressor/refrigerant runs might prove to be a critical feasibility/cost-limiting factor in application, relative to a simpler closed hydronic loop (circulating water over long distances can be easier). I guess I?m a biased fan of runarounds ? but don?t discount the potential to really optimize savings of existing systems with proper controls/sequencing. These are often not operated/installed as intended/designed (just circulating 24/7), so even if something is already in place you might have some low hanging fruit to grab there ?.

**Tangent Alert**

I?m in the middle of a project to implement a souped-up version of exhaust heat recovery, but not for buildings. Application is a series of large existing steam boiler plants (Combustion stack economizers)! This will entail installing heat recovery coils (and bypassing infrastructure) onto existing large boiler plant exhaust gas stacks. We can then capture the heat in two stages: First stage pulls boiler exhaust gas (~400F to atmosphere) down ~100F with a loop to carry that heat into the deaerator process of the boilers. Second stage (condensing) economizer cools the combustion gasses further (~130F to atmosphere), applying that energy to pre-heat makeup/feedwater coming into the boilers.

Thought it might be interesting for some to cross-connect the potential for heat recovery systems in a broader sense. My experiences with deconstructing and optimizing typical building mechanical systems was definitely a bridge for figuring out how to approach and do the same for hydronic plants ? this is one of those sorts of mental bridges that helped me to get where I am.

**Flailing effort to get back on point Alert**

I wasn?t the best student through my chemistry courses decades ago, but I have a sneaky feeling that sticking a typical refrigerant filled line/coil in a boiler exhaust gas stream with hopes of recovering heat might just pose a small risk of violent explosions and total catastrophe? I?ll leave that one for the rocket scientists among us to chew on =).

~Nick

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Nicholas, I think this is one big tangent! But your point illustrates that heating is as much a heating process as it is a cooling process. The traditional laws of thermodynamics (I did have to check them) don't describe this in an easy to understand way... So I'll just call it "Newton's 3rd law, but for thermodynamics".

Regarding potentially long refrigerant runs. We see refrigerant dual purposed as the heat transfer medium in heat-pump AHU's such as the Flakt Woods model I linked to earlier. However, I'd like to consider "dual purposing" the CHW loop for heat recovery in winter. In Europe we are much more familiar with piping chilled water around buildings to terminal units. So, distributing heat, cold and recovery via 4 pipes to remote AHUs probably won't be so much of an issue.

There is also the potential to push low grade recovered heat back to recharge ground-source boreholes. By doing this, a smaller borehole array may be sufficient and heat pumps may operate at a more consistently high COP. Recharging boreholes shouldn?t take any compressor power, just pumping.

Based on some other conversations, the low gas costs and high capex of these systems are a barrier.

Cheers

Chris

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Chris,

This is a similar product that we used in a hospital in Germany that I?m told are very common.
https://www.flaktgroup.com/en/solutions/maintaining-hygienic-conditions/econot-product-solution/
http://resources.flaktwoods.com/Perfion/File.aspx?id=7b7e19a7-0862-4a74-a9cf-97dbe9a120e8

It?s worth noting in Germany hospitals are 100% outside so they are more laboratory like, but in general move less total air than US hospitals.

I don?t think the FW product is quite as efficient as the Konvekta solution, but it?s cheaper.

[Doing this from memory from ~4 years ago] Per regulations in Germany I believe air-side heat recovery effectiveness needs to be above 60% or 65% so a standard run around loop won?t achieve that. There?s also regulations regarding heat recovered per unit fan energy used so you?re not chewing up all your savings with static pressure.

Not sure how uniform Europeans are on their energy code from country to country.

AEI has done the system you?re describing in laboratories several years back with a field built solution tying in the condenser or evaporator side of the heat pump into the run around loop depending on climate and load profile. It was pretty effective in places like San Francisco, Washington DC, etc. [not extremely hot or cold] so you get more run hours out the RAL due to better approach temperatures and can support process cooling or reheat. I modeled this in TRNSYS as eQUEST doesn?t bend that way. We weren?t really using e+ back then so I didn?t try this configuration in that software.

Getting the controls to work in the field was another matter so there?s a lot of benefit to a packaged solution like the ones identified in this thread. You can then integrate the heat pump anywhere in the chilled/hot water loops (assuming appropriate temperatures) and the heat recovery controller will dispatch according to what?s best for the RAL while loading the heat pump or heat recovery chiller as needed.

Hope that helps.

Fred

Fred Betz PhD, LEED? AP BD+C
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?Hi Chris,

I believe the system you you are describing has been utilized in North American hospitals. There was an article in the June 2014 edition of ASHRAE Journal - Hydronic Heat Recovery in Healthcare - which describes this type of system. Refer to the attached schematic from the article. It is also mentioned in ASHRAE?s 50% Savings Advanced Energy Design Guide for Large Hospitals. I?ve read about it being integrated into a few hospital projects since then (subsequent ASHRAE articles) and we are working on some ourselves now.

There is a Canadian AHU manufacturer (AIR) providing this type of system as part of a packaged AHU but it loses one of the major benefits of taking a larger systems approach which the article describes - where you to recover heat from the building exhaust stream (from lights, equipment, people, etc) and use that for the largest energy use in a (North American) hospital - reheat. The heat recovery chiller should achieve a COP of ~4.0 year around which is a pretty economical heating source with the side benefit of ?free? chilled water. I think it provides over 90% of the annual heating in the hospital described in the article.

Hope this helps,
Aaron

Aaron Smith, P.Eng
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Aaron, Fred

Thanks. This is very helpful. Found the AJ article here: https://anthraciteashrae.weebly.com/uploads/2/2/8/5/22852420/hydronic_heat_recovery_in_health_care.pdf

Cheers

Chris

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There are a number of buildings here in WA State/Seattle that are incorporating a central HR chiller and/or heat pumps, and incorporating this exhaust air heat recovery coil as part of that design. Primarily hospitals and labs, but we have even seen this strategy used in an office building that uses a large DOAS. In that case, the coil satisfies our code requirement that DOAS systems have energy recovery. That particular building also has the ability to change over flow from the chilled water to hot water, and reject heat to the exhaust air, expanding the number of hours that the HR chiller can be used meet all of the building loads.

In these cases, we have always just modeled the building in eQUEST or EnergyPlus, taken the hourly loads on the loop, and modeled the plant outside the program using a big spreadsheet.

As of 2/1/21, this strategy will be required by in the WA state energy code , primarily targeted at hospitals (see below a snipet of section the WA amendment C403.9.2.4).

David Reddy P.E. he/him/his

Building Performance Principal

Tel 206-621-8626 x201
Cell 206-406-9856

Fax 206-621-8649

O?Brien360?710 Second Ave., Suite 925

Seattle, WA 98104

david at obrien360.com

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David

I was considering how the exhaust may be dual proposed for heat rejection,
but the idea was having a hard enough time gaining traction. I also
(rightly or wrongly) have a dislike of "changeover modes", preferring
things to work in a much more continuous manner.

Thanks

Chris

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