Earth, Air, Water, and Ice.

In my attempts at Ice Storage Heat Source popularization I have been facing one big challenge: How can you – succinctly, using pictures – answer questions like:

How much energy does the collector harvest?


What’s the contribution of ground?


Why do you need a collector if the monthly performance factor just drops a bit when you turned it off during the Ice Storage Challenge?

The short answer is that the collector (if properly sized in relation to tank and heat pump) provides for about 75% of the ambient energy needed by the heat pump in an average year. Before the ‘Challenge’ in 2015 performance did not drop because the energy in the tank had been filled up to the brim by the collector before. So the collector is not a nice add-on but an essential part of the heat source. The tank is needed to buffer energy for colder periods; otherwise the system would operate like an air heat pump without any storage.

I am calling Data Kraken for help to give me more diagrams.

There are two kinds of energy balances:

1) From the volume of ice and tank temperature the energy still stored in the tank can be calculated. Our tank ‘contains’ about 2.300 kWh of energy when ‘full’. Stored energy changes …

  • … because energy is extracted from the tank or released to it via the heat exchanger pipes traversing it.
  • … and because heat is exchanged with the surrounding ground through the walls and the floor of the tank.

Thus the contribution of ground can be determined by:

Change of stored energy(Ice, Water) =
Energy over ribbed pipe heat exchanger + Energy exchanged with ground

2) On the other hand, three heat exchangers are serially connected in the brine circuit: The heat pump’s evaporator, the solar air collector, and the heat exchanger in the tank. .

Both of these energy balances are shown in this diagram (The direction of arrows indicates energy > 0):

Energy sources, transfer, storage - sign conventions

The heat pump is using a combined heat source, made up of tank and collector, so …

Ambient Energy for Heat Pump = -(Collector Energy) + Tank Energy

The following diagrams show data for the season containing the Ice Storage Challenge:

Season 2014 - 2015: Monthly Energy Balances: Energy Sources, Transfer, Storage

From September to January more and more ambient energy is needed – but also the contribution of the collector increases! The longer the collector is on in parallel with the heat pump, the more energy can be harvested from air (as the temperature difference between air and brine is increased).

As long as there is no ice the temperature of the tank and the brine inlet temperature follow air temperature approximately. But if air temperature drops quickly (e.g. at the end of November 2014), the tank is still rather warm in relation to air and the collector cannot harvest much. Then the energy stored in the tank drops and energy starts to flow from ground to the tank.

2014-09-01 - 2015-05-15: Temperatures and ice formation

2014-09-01 - 2015-05-15: Daily Energy Balances: Energy Sources, Transfer, Storage

On Jan 10 an anomalous peak in collector energy is visible: Warm winter storm Felix gave us a record harvest exceeding the energy needed by the heat pump! In addition to high ambient temperatures and convection (wind) the tank temperature remained low while energy was used for melting ice.

On February 1, we turned off the collector – and now the stored energy started to decline. Since the collector energy in February is zero, the energy transferred via the heat exchanger is equal to the ambient energy used by the heat pump. Ground provided for about 1/3 of the ambient energy. Near the end of the Ice Storage Challenge (mid of March) the contribution of ground was increasing while the contribution of latent energy became smaller and smaller: Ice hardly grew anymore, allegedly after the ice cube has ‘touched ground’.

Mid of March the collector was turned on again: Again (as during the Felix episode) harvest is high because the tank remains at 0°C. The energy stored in the tank is replenished quickly. Heat transfer with ground is rather small, and thus the heat exchanger energy is about equal to the change in energy stored.

At the beginning of May, we switched to summer mode: The collector is turned off (by the control system) to keep tank temperature at 8°C as long as possible. This temperature is a trade-off between optimizing heat pump performance and keeping some energy for passive cooling. The energy available for cooling is reduced by the slow flow of heat from ground to the tank.

Frozen Herbs and Latent Energy Storage

… having studied one subject, we immediately have a great deal of direct and precise knowledge … of another.

Richard Feynman

Feynman referred to different phenomena that can be described by equations of the same appearance: Learning how to calculate the distribution of electrical charges gives you the skills to simulate also the flow of heat.

But I extend this to even more down-to-earth analogies – such as the design of a carton of frozen herbs resembling our water-tight underground tank.

(This is not a product placement.)

No, just being a container for frozen stuff is too obvious a connection!

Maybe it is the reclosable lid covering part of the top surface?

Lid of underground water/ice storage tank.

No, too obvious again!

Or it is the intriguing ice structures that grow on the surface: in opened frozen herb boxes long forgotten in the refrigerator – or on a gigantic ice cube in your tank:

Ice Storage Challenge of 2015 - freezing 15m3 of water after having turned off the solar/air collector.

The box of herbs only reveals its secret when dismantled carefully. The Chief Engineer minimizes its volume as a dedicated waste separating citizen:

… not just tramping it down (… although that sometimes helps if some sensors do not co-operate).

He removes the flaps glued to the corners:

And there is was, plain plane and simple:

The Chief Engineer had used exactly this folding technique to cover the walls and floor of the former root cellar with a single piece of pond liner – avoiding to cut and glue the plastic sheet.

We Want Ice!

We haven’t seen much of it this winter yet.

I am talking both about the ice you would expect in winter and about the one created from extracting heat from a water tank – our heat pump system‘s heat source.

This winter does again disappoint; it seems we will not be able to generate Pannonia‘s largest ice cube in this season. This plot shows the growth of ice in the past three seasons, since the system went live in autumn 2012:

Energy stored in the water tank, January 2015The tank of water can be considered a buffer that stores energy harvested by the solar collector; in addition some energy is directly harvested from the surrounding ground.

The water tank temperature is 20°C maximum. This is the maximum heat source temperature the heat pump can deal with, so the solar collector is hardly used in summer. Heat provided by ground is sufficient to provide the energy which is extracted from the tank on heating hot water.

This is the energy stored in the tank over time:

Energy stored in the water tank-2015-01The specific heat of water is 1,16kWh per m3  – cooling down the 25m3 tank from 20°C to 0°C provides about 580kWh. Currently we need about 70kWh per day for space heating and hot water heating; the maximum in this season was about 100kWh per day so far. We had not seen ice before December in the past three seasons: Water does not freeze as long as as the energy provided by the solar collector replenishes the energy in the tank quickly enough.

The ice formation curves in the first figure show that the blue peaks always follow a cold spell of weather –  a negative peak in the (green) ambient temperature. As soon as there is a positive peak the ice is quickly melted again. This year the latest green positive peak was quite pronounced – about 12°C average daily temperature; maximum temperatures were about 20°C in some regions in Austria.

But we try harder now to create a gigantic ice cube: On rebuilding the solar collector last summer a new feature has been added for research purposes – the effectively utilized area of the collector can be changed by letting brine only flow through a subset of the tubes.

Currently we use only the upper half of the area. There is hoarfrost on the pipes which are in use – as they are colder as energy is extracted from the flowing brine by the heat pump and / or by the water tank:

Solar collector in winter, half of the area used

If this is still not sufficient to challenge the system we might turn off the collector permanently in February. 100kWh heating energy per day translates to 75kWh to be extracted by the heat pump (given a performance coefficient of about 4). The tank containing about 2.000 kWh would then be exhausted and completely frozen in 27 days.

Solar collector in winter, half the area used, closeup


Further information:

Other plots and key performance data for each month and each season are detailed in our documentation of measurement data – this file contains two full seasons as per the writing of this blog post.

In the unlikely case somebody stumbles upon this post when searching for historical weather data for Austria: The English Annals page show the data in a format that is difficult to work with (you need an outdated browser), but CSV files can be downloaded from the German page with historical data. Pick daily data (Tagesauswertung) for the greatest level of detail.

Biology / Chemistry Challenge or: Should We Really Blame the Dead Frog?

We often say we operate in Leonardo da Vinci Renaissance Mode – given our odd ‘portfolio of diverse services’. But as much as the Chief Engineer does not like to work with mortar, cement, or any other slimy substances I tried to avoid pondering about the intricacies of living beings and chemicals so far.

But slimy, smelly species were to strike back.

For two years we could confirm with confidence that the water in our underground water tank / heat sink does never smell. Until two weeks ago when the water appeared a bit turbid and there was the signature ‘rotten eggs’ smell of hydrogen sulfide.

Water in ponds can ‘die’ – due to eutrophication: Algae bloom due to too much nutrients, die, and their decomposition by bacteria consumes all the oxygen in the water. This can kill fish and other species who need the oxygen.

Since the tank is dark there are no algae but there might be other biomass, subject to decomposition. The most recent rebuilding of the solar collector and brine piping has probably invited some curious or suicidal life-forms.

Dead frog or toad

Researching cisterns in a German forum (‘The water in our cistern stinks’) I learned the following:

“Clean your gutters and check your filters.”

“Install an air pump to supply the bacteria in the water with oxygen. Otherwise there will be anaerobic decomposition, such as turning sulfate into sulfide. The pump resolved this issue within two days.”

“Use chlorine, as for swimming pools.”

“Absolutely don’t use chlorine – cisterns are designed to work without intervention. We use a cistern for twenty years and never had to use chemicals.”

“If the black layer of mud at the bottom of the cistern gets too thick there will be anaerobic decomposition.”

“The black layer of mud at the bottom of the cistern is like a natural sewage plant.”

“Our cistern smelled, too, and we found six dead birds at its bottom.”

Insights are somewhat contradictory but there were more accounts of people advocating the additional supply of oxygen and otherwise letting the bacteria do their work. Proper filters and sealings should prevent the invasion of animals of course.

We still wondered about the coincidence of the H2S accident and recent repairs – was it only due to a sudden invasion of (more) worms, snails, and frogs because of some pipes that were open for a short period of time?

But several users reported that a small amount of brine from their solar collectors had trickled into their cisterns and gave rise to the rotten eggs smell

With cisterns brine could be collected by the gutters, from leaky collectors mounted at the rooftop.

And yes – we had a leak in the new part of the brine piping. The Chief Engineer had heroically cleaned the emptied tank and used the ‘synergy effects’ of being able to do some other maintenance.

Chief Engineer in the Tank

A few days after having refilled the tank the water showed turbid streaks again – and we finally spotted the leak in the new part of the brine piping. Now it is leakproof again!

The good thing finally is:

  • It all boils down to simply following ‘cistern best practices’.
  • The smell provides for a sensitive early warning systems that signals a leak in brine piping.

And we have a new gadget now:

Air Pump

Why is brine so detrimental? Brine used for solar collectors contains about 40% ethylene glycol – frost protection. This provides a feast for bacteria – like sugar. On airports the fluid used for de-icing airplanes is collected and then decomposed by bacteria in a biological sewage plant. It seems that in a tank bacteria reproduce like hell, die, and are finally decomposed anaerobically when there is no more oxygen left.

Tiny amounts of brine alone seem to be suffcient to trigger that chain reaction within days – whereas the occasional earth worm did not do any harm in the past years.

We hope we will be able to keep the right variety of bacteria happy in the future rather than fight them. However, as a child of the 1970s and fan of typical related cartoons and commercials I cannot imagine them other than this:

Fight Bac! (085 086) (7396068296)Edit: As I have asked about the treatment systems on airports: Treating glycol runoff from airport deicing operations

Controlling the Four Elements. Or: Why Heat Pumps Are Cool.

Despite my attempts to post mainly geeky and weird stuff peppered with (very often not down-to-earth) physics, I got involved in some serious discussions on renewable energy, sustainability, heat pumps, and the pleasures of Building Your Own Stuff. So I will describe what I am actually working on / playing with when I am not blogging, liking, sharing or tweeting.

Elkement is an amalgam of my first name, Elke, and one of my nicknames, The Subversive Element. I have a penchant for the words elementary and elemental, any puns comprising those, and I like geek gadgets exhibiting the periodic system of elements. It is straight-forward that I have to work on a heating system that utilizes The Four Elements: Earth, Wind, Air and Fire.

As usual, I first considered this an incredibly creative way to describe a heating system – until I discovered that zillions of companies in the HVAC business use the same analogy. Besides I like the philosophical, if not new age-y, connotation contrasting with down-to-earth engineering.

I (we) work on optimizing a heat pump system that uses these said elements, which actually is: An unconventional source of heat that makes the system different from geothermal, ground water or air source heat pumps.

This is a simplified schema. Please bear with me!

The Four Elements are indicated: A rather large (about 20 m3) tank filled with Water is heated using an unglazed solar collector. This collector allows for harvesting energy from solar radiation (Fire), but – above all – from ambient Air via convection.

The tank is located below ground (Earth). Actually, in the prototype system the ‘tank’ is a former small cellar that once had been used to store potatoes and wine (This is common in the region where I live). The cellar had been lined with plastic sheets which makes it a pretty cheap tank. There is an average net flow of energy from the ground to the tank – it would be detrimental to insulate the bottom and the side walls of the tank.

Heat pump system utilizing a water (ice) tank as heat source (Copyright:

Heat pump system utilizing a water tank as heat source. Energy is harvested from ambient air, solar radiation, heat transfer from the ground, and from freezing water (Copyright:

The water tank constitutes the heat source: Instead of burying pipes in the garden (ground source loops), the heat exchanger pipes are immersed in the tank.

It is a single (simple) closed cycle:

Heat pump –> solar collector –> tank –> heat pump

The combination of the tank and the collector is the actual heat source of the heat pump.

Depending on the temperature difference between ambient air and the tank and on the heating demand of the building the controller decides whether the collector is used or circumvented and whether the heat pump is turned on.

The heat pump heats two different hot water tanks – one is for heating the tap water, the other one is for transferring heat to the room heating loops. A heat pump cannot be “dimmed” continuously to different output powers: It delivers heat at full power or it is off. Thus you need an intermediate storage that allows for gradual heat transfer to the heating loops.

There is an important 5th element (You have expected me to add a link like that, didn’t you?): Ice.

During the heating season about 75% of the heating energy is harvested from air and fire channeled through the solar collector, freezing of the water in the tank and heat transfer from the ground yields the remaining 25%.

But heat pumps are cool, and this setup allows for a simple way of passive (“free”) cooling: In summer the room heater becomes a cooler: The ‘Hot Water Tank for Room Heating’ becomes the heat exchanger that transfers heat from the room heating loops to the underground tank. The heat pump can still heat the tap water in parallel – actually this is beneficial for cooling!

The overall goals for this design have been:

  • Using a heat pump with a high seasonal performance factor, but avoiding the drilling of deep bore holes or having to turn the whole area of your garden into a huge pile of soil temporarily.
  • Using rather simple, state-of-the-art components that could be purchased by home owners in online shops and that would allow skilled Do-It-Yourself enthusiasts to built the system themselves.
  • Allowing for cooling without adding complicated components or needing air-condition (to American readers: AC in homes in uncommon in Europe).


Further reading: This was the pre-quel of the story (“How I started loving heat pumps more than IT.”)
Our German website comprises very detailed technical information – I am not sure of we will ever provide the same level of details in English.