Heat Pump And Heat Exchanger For Heating And Cooling

[redcane]

G'day All,

I've been doing a lot of reading about the automated rigs that various brewers have constructed, and a thought popped into my head. The brewing process requires heating a tank of water to various degrees, ultimately to a rolling boil (by adding heat energy) and then cooling as quickly as possible (by removing heat energy). So what if the heat energy pumped into the wort could be taken from a tank of (now) cooled water, and that cooled water could then be used to chill the wort after the boil?

I'm thinking of a system like the heat pump on a hot water system, except that the cold side would be run into a cold tank heat exchanger, so as you heat up a hot liquor tank, you are simultaneously cooling the cold tank. Depending on the coefficient of performance of a heat pump you'll also get more than 1 watt of heat into your wort for each 1 watt of electricity consumed (some heat exchangers can transfer 6 watts of heat per 1 watt of electrical input). So in theory this should allow us to heat the wort with less energy than a normal heating element and you also get a store of chilled water. Then add a heat exchanger in the wort tank, then pump either the hot tank water or cold tank water through that in order to heat or cool the wort.

Disadvantages - heat pumps are much more expensive than element heaters, the maximum heating capacity might be lower, and you likely can't heat to boiling (as far as I know).

It seems like you could turn the heat pump on in advance of the brew to give it time to heat/cool the hot/cold tanks to get around the lower maximum heating capacity of the heat pump (might want the tanks insulated). You'd probably need an electric element to boost to boiling temp (but you'd be starting from a higher than ambient temp so it shouldn't take long). Can't do much about the disadvantage of cost though.

Anyone see any reason why this wouldn't work? Maybe people could use this to keep the brewery electricity bills down, I'd be tempted to give it a try once I get to the stage of building a more permanent brewery.

 

[Wolfy]

Pretty sure most commercial breweries do it that way - as the wort is cooled via heat-exchanger from the kettle - it heats the water for the next mash-in, even many home brewers use the heated (plate or CFC heat exchanger) waste water to wash things up with.
The problem with the concept on a home-scale is that it requires running multiple consecutive brews, and I think for most that brewing one beer a day is enough.

 

[seravitae]

I had this idea ages ago and was tempted to implement it into a brewery product.

Unfortunately it is too expensive on this scale. Even on the large breweries, as far as I understand, heat exchangers are used, but not heat pumps.

The only hot water heat pumps that can even get close to what we need for brewing are the supercritical CO2 based heat pumps, that push up to 80C water. But this is still not high enough for a boil. And you cannot supplement the heat pump with a resistive heater, as the coil will be subjected to up to 100C which it may not be designed for and may damage the system.




There is absolutely no doubt that if you are doing multiple batches, that from your kettle you could 'chill' your wort with fresh water into your HLT for the next batch. It can help financially but usually not on our scale.

Also important to realize that without active pumping (heat pump), that the temperatures at best will achieve thermal equilibrium, meaning that if you're expecting your 100C wort to cool to 20C and your 20C HLT water to go to 100C, you'll be disappointed.

If you are interested in how much it would potentially save you in money, please list your batch size, I will do the equations (to which I believe you will be dissapointed with the savings).

Cheers,

Seb

 

[redcane]

It certainly makes sense that if you are doing multiple batches you use heat dumped into the chill water to pre-heat the next batch. I'm surprised on a large scale breweries wouldn't be using a ground source heat pump (or similar) as well to reduce their bills. Even just pre-heating with a solar thermal hot water system should have a fairly quick pay off I would think?

In terms of the drawbacks of my idea I hadn't looked into the temperature range of heat pumps - but I know that our hot water system would heat to over 60 degrees Celsius (extracting heat from the surrounding air even on nights that got down to zero). I would think even this level of heat would be useful, even if it's just used for preheating. As for not being able to supplement with a resistive heater in case of damage to the coil, my plan was to have the resistive heater in the wort tank, and the heat pump would pump heat into a separate hot tank, then either re-circulate the hot tank water through the wort or vice versa (whatever is easiest to clean I guess). That way only the wort tank portion is exposed to boiling temperatures, not the heat pump itself.

Do you have any idea of the co-efficient of performance for a realistic heat pump that is working over this temperature range? I know some commercially available reverse cycle air conditioners get up to almost 600%. Some back of the envelope calcs make me think we can save 40% of the electricity. Assuming we start with water at 20 degrees, and can take it to 60 with the heat pump, then have to heat to 100 with the resistive heater it appears we can do 50% of the heating with the heat pump. Assuming the heat pump uses 1/6th of the electricity we are using 1/6th of the electricity for 50% of the boil and 100% for 50%, so (100%*.5) + (.5 *1/6) = ~58.3%.

I think however a larger proportion of the heating takes place at 100 degrees for the boil (due to the phase change)? This will lower the percentage of heating that can be done by heat pump. It's also a fairly optimistic co-efficient of performance since it's a much higher temperature differential than the reverse cycles that have that co-efficient. The savings could be higher if we can get the heat pump to a higher water temp, but we also have energy usage in the pumps to re-circulate the water. (This is ignoring the benefit of having chilled water as well). Unless there is something drastically wrong with my thinking, or my assumed figures are completely wrong a saving of up to 40% seems like it could be worthwhile.

I guess the next part of the maths is how much the power is costing to heat the batch to work out potential savings, then the cost of making it happen to work out how many batches before it'd pay off.

I don't have a particular batch size in mind yet - I'm not at the stage of building a 'proper' brewery yet, but one day when I get around to it I'll have to figure out more accurately if this would be worthwhile.

 

[QldKev]

I don't think you will find a lot of people on here would have considered doing it, for the simple reason batch sizes. For a homebrewer batch they are normally around 23L, and have a total elec cost of around $1. As a single batch brewer would stand to save under 50c per brew from the change. In my case a under $2 per month saving.

Even if you had a 100x system. At 2300L the saving would still only be under $50 per brew. Then what would be the cost of a heat exchanger capable of the water requirements, and how long would be the pay back period. I don't think a system that took too long, delaying the next brew would be a good option, but that would depend in the brewery plans. If you are setting the gear up from scratch for a large scale, then a heat pump Vs elec or gas heating system cost benefit analysis could be done.

In the home we are getting more and more heat pumps for our hot water, so filling the HLT from the HWS for both the initial water and the sparge water will save some power, but nothing dramatic. Although in saying that I did use my HWS for a couple of brews, and could detect a difference in the final brew. I could never work out why as the water from the HWS tasted ok, but other people commented on the change in the final beer without knowing I had changed water sources. But to be fair it was an old elec storage water system.

Recircing waste heated water from the chiller back into the HLT for another batch is a great option, and some commercial breweries are doing it.

QldKev

 

[MHB]

Energy is one of the major costs in commercial brewing (dearer than malt and hops), I spent a lot of time researching this a couple of years ago and just from memory. You can get about an 80% energy saving with a well designed energy fold back system.
A couple of points you might find useful:
The biggest product produced by breweries is Warm Water (not hot enough to heat with, too hot to cool with). Interestingly enough water is cheap dumping it down the sewer is not!
There is a lot of energy in kettle steam (look at heat of evaporation) vapour condensers can get most of this back and heat warm water into hot water.
There is a lot of wasted heat in flue gases; again a lot can be recovered.
Multi stage heat exchangers are a lot more efficient than single stage.
Good insulation pays for itself very quickly.

Modern brewery design is really mostly about energy conservation with in the process, where it all comes together is if you are brewing constantly, on a batch/week or so there isnt a lot you can do and accumulating the energy is only useful if you use it soonish.
I did look at both solar and reverse cycle, at the time you could get 80% saving with conventional processes and maybe another 10% with advanced methods; the saving simply didnt repay the investment over the life of the plant.
As the cost of energy and water (malting and brewing are profligate users of both) rise there will be a time when the investment becomes well worthwhile.
It will be a lot longer before home brewers reach that point, better to look at savings in more conventional ways, electric heating and good insulation being at the fore.
This doesnt mean I dont applaud all reasonable steps to save on energy; just not sure that reverse cycle is the way to go in a small batch brewery, well not yet anyway.
Mark

 

[redcane]

Ok, that's interesting - I thought the electricity costs might be higher than that, but I've never measured. It's going to take a lot of brews at a saving of 50c a piece to pay any investment in parts off.

When I do the (rough) math on the heat energy you need it does come in at a low figure. Heating 23 kg of water by 100 degrees should take 2,300,000 calories (not allowing for phase change), and that is only about 2.7 kilowatt hours, which even with the highest electricity prices should only be 80c. I guess adding a third for heat loss etc and you might get to about $1. (Should have done this math first before I let my mind race off!)

I guess that brings the advantages of my system down to just that of having chilled water for cooling the wort at the end of a batch, and even then you are going to pay for it up front. You can effectively hire out this system by buying bags of ice that someone else has heat-pumped the energy out of.

Part of the reason I was somewhat interested in this was that I might be living with an off-grid solar system in the future. So I'll probably either need to wait for sunny days to do brewing, or reduce electricity usage, or buy more solar panels. Alternatively I might just have to cook with gas instead.

Thanks for the responses! It's good to have joined a community of knowledgeable brewers!

 

[Wortgames]

Also important to realize that without active pumping (heat pump), that the temperatures at best will achieve thermal equilibrium

With a counterflow heat exchanger you can get almost a total heat exchange, as the cooled wort exits at almost the same temp as the cooling water enters, and the heated cooling water exits at almost the same temp as the hot wort enters.

I realise this has nothing at all to do with heat pumps, but a well-designed heat exchange system can be surprisingly effective. I get scalding hot water out of mine and chill a 45-litre batch from boiling to pitching in about 15 mins, using dam water that isn't especially cold to begin with :super: