The Heat Source Paradox

It is not a paradox – it is a straight-forward relation between a heat pump system’s key data:

The lower a heat pump’s performance factor is, the smaller the source can be built.

I would not write this post, hadn’t I found a version of this statement with a positive twist  used in an advert!

In this post I consider a heat pump a blackbox that converts input energy into output heat energy – it ‘multiplies’ energy by a performance factor. A traditional mechanical heat pump uses electrical input energy to drive a mechanical compressor. The uncommon Rotation Heat Pump utilizes the pressure gradient created by centrifugal forces and thus again by electrical power.

But a pressure difference can also be maintained by adsorption/desorption processes or by changing the amount of one fluid dissolved in another; Einstein’s famous refrigerator uses a more complex combination of such dissolution/evaporation processes. Evaporation or desorption can be directly driven by heat: A gas heat pump thus ‘multiplies’ the energy from burning natural gas (and in addition, a heat pump and a gas boiler can be combined in one unit).

The overall performance factor of a gas heat pump – kWh heating energy out over kWh gas in – is about 1,5 – 2. This is lower than 4 – 5 available with mechanical compressors. But the assessment depends on the costs of kWh gas versus kWh electrical energy: If gas is four times cheaper (which nearly is the case in Germany) than burning natural gas in a traditional boiler without any ‘heat pump multiplication’, then the classical boiler can be more economical than using a heat pump with an electrical compressor. If gas is ‘only’ two times as cheap, then a gas heat pump with an overall performance number of ‘only’ 2 will still beat an electrical heat pump with a performance factor of 4.

While the gas heat pump may have its merits under certain market conditions, its performance number is low: For one kWh of gas you only get two kWh of heating energy. This  means you only need to provide one kWh of ‘ambient’ energy from your source – geothermal, water, or air. If the performance factor of an electrical heat pump is 4, you multiply each kWh of input energy by 4. But the heat source has to be able to supply the required 3 kWh. This is the whole ‘paradox’: The better the heat pump’s performance is in terms of heating energy over input energy, the more energy has to be released by a properly designed heat source, like ground loops sufficiently large, a ground-water well providing sufficient flow-rate, an air heat pump’s ventilator powerful enough, or our combination of a big enough solar/air collector plus water tank.

Illustration of the ‘heat source paradox’: The lower the performance number (ratio of output and input energy), the lower is the required ambient energy that has to be provided by ‘the environment’. The output heating energy in red is the target number that has to be met – it is tied to the building’s design heat load.

If you wish to state it that way, a heat pump with inferior performance characteristics has the ‘advantage’ that the source can be smaller – less pipes to be buried in the ground or a smaller water tank. And in an advert for a gas heat pump I found it spelled out exactly in this way, as a pro argument compared to other heat pumps:

The heat source can be built much smaller – investment costs are lower!

It is not wrong, but it is highly misleading. It is like saying that heating electrically with a resistive heating element – and thus a performance number of 1 – is superior because you do not need to invest in building any source of ambient energy at all.

Data for the Heat Pump System: Heating Season 2016-2017

I update the documentation of measurement data [PDF] about twice a year. This post is to provide a quick overview for the past season.

The PDF also contains the technical configuration and sizing data. Based on typical questions from an ‘international audience’ I add a summary here plus some ‘cultural’ context:

Building: The house is a renovated, nearly 100-year old building in Eastern Austria: a typical so-called ‘Streckhof’ – an elongated, former small farmhouse. Some details are mentioned here. Heating energy for space heating of two storeys (185m2) and hot water is about 17.000-20.000kWh per year. The roof / attic had been rebuilt in 2008, and the facade was thermally insulated. However, the major part of the house is without an underground level, so most energy is lost via ground. Heating only the ground floor (75m2) with the heat pump reduces heating energy only by 1/3.

Climate: This is the sunniest region of Austria – the lowlands of the Pannonian Plain bordering Hungary. We have Pannonian ‘continental’ climate with low precipitation. Normally, monthly average temperatures in winter are only slightly below 0°C in January, and weeks of ‘ice days’ in a row are very rare.

Heat energy distribution and storage (in the house): The renovated first floor has floor loops while at the ground floor mainly radiators are used. Wall heating has been installed in one room so far. A buffer tank is used for the heating water as this is a simple ‘on-off’ heat pump always operating at about its rated power. Domestic hot water is heated indirectly using a hygienic storage tank.

Heating system. An off-the-shelf, simple brine-water heat pump uses a combination of an unglazed solar-air collector and an underwater water tank as a heat source. Energy is mainly harvested from rather cold air via convection.

Addressing often asked questions: Off-the-shelf =  Same type of heat pump as used with geothermal systems. Simple: Not-smart, not trying to be the universal energy management system, as the smartness in our own control unit and logic for managing the heat source(s). Brine: A mixture of glycol and water (similar to the fluid used with flat solar thermal collectors) = antifreeze as the temperature of brine is below 0°C in winter. The tank is not a seasonal energy storage but a buffer for days or weeks. In this post hydraulics is described in detail, and typical operating conditions throughout a year. Both tank and collector are needed: The tank provides a buffer of latent energy during ‘ice periods’ and it allows to harvest more energy from air, but the collector actually provides for about 75% of the total ambient energy the heat pump needs in a season.

Tank and collector are rather generously sized in relation to the heating demands: about 25m3 volume of water (total volume +10% freezing reserve) and 24m2 collector area.

The overall history of data documented in the PDF also reflects ongoing changes and some experiments, like heating the first floor with a wood stove, toggling the effective area of the collector used between 50% and 100%, or switching off the collector to simulate a harsher winter.

Data for the past season

Finally we could create a giant ice cube naturally. 14m3 of ice had been created in the coldest January since 30 years. The monthly average temperature was -3,6°C, 3 degrees below the long-term average.

(Re the oscillations of the ice volume are see here and here.)

We heated only the ground floor in this season and needed 16.600 kWh (incl. hot water) – about the same heating energy as in the previous season. On the other hand, we also used only half of the collector – 12m2. The heating water inlet temperatures for radiators was even 37°C in January.

For the first time the monthly performance factor was well below 4. The performance factor is the ratio of output heating energy and input electrical energy for heat pump and brine pump. In middle Europe we measure both energies in kWh ;-) The overall seasonal performance factor was 4,3.

The monthly performance factor is a bit lower again in summer, when only hot water is heated (and thus the heat pump’s COP is lower because of the higher target temperature).

Per day we needed about 100kWh of heating energy in January, while the collector could not harvest that much:

In contrast to the season of the Ice Storage Challenge, also the month before the ‘challenge’ (Dec. 2016) was not too collector-friendly. But when the ice melted again, we saw the usual large energy harvests. Overall, the collector could contribute not the full ‘typical’ 75% of ambient energy this season.

(Definitions, sign conventions explained here.)

But there was one positive record, too. In a hot summer of 2017 we consumed the highest cooling energy so far – about 600kWh. The floor loops are used for passive cooling; the heating buffer tank is used to transfer heat from the floor loops to the cold underground tank. In ‘colder’ summer nights the collector is in turn used to cool the tank, and every time hot tap water is heated up the tank is cooled, too.

Of course the available cooling power is just a small fraction of what an AC system for the theoretical cooling load would provide for. However, this moderate cooling is just what – for me – makes the difference between unbearable and OK on really hot days with more than 35°C peak ambient temperature.

Heat Transport: What I Wrote So Far.

Don’t worry, The Subversive Elkement will publish the usual silly summer posting soon! Now am just tying up loose ends.

In the next months I will keep writing about heat transport: Detailed simulations versus maverick’s rules of thumb, numerical solutions versus insights from the few things you can solve analytically, and applications to our heat pump system.

So I checked what I have already written – and I discovered a series which does not show up as such in various lists, tags, categories:

[2014-12-14] Cistern-Based Heat Pump – Research Done in 1993 in Iowa. Pioneering work, but the authors dismissed a solar collector for economic reasons. They used a steady-state estimate of the heat flow from ground to the tank, and did not test the setup in winter.

Cistern-Based Water-Source Heat Pump System Design, 1993[2015-01-28] More Ice? Exploring Spacetime of Climate and Weather. A simplified simulation based on historical weather data – only using daily averages. Focus: Estimate of the maximum volume of ice per season, demonstration of yearly variations. As explained later (2017) in more detail I had to use information from detailed simulations though – to calculate the energy harvested by the collector correctly in such a simple model.

Simple simulations of volume of ice[2015-04-01] Ice Storage Challenge: High Score! Our heat pump created an ice cube of about 15m3 because we had turned the collector off. About 10m3 of water remained unfrozen, most likely when / because the ice cube touched ground. Some qualitative discussions of heat transport phenomena involved and of relevant thermal parameters.

Ice formation during the 'ice storage challenge'[2016-01-07] How Does It Work? (The Heat Pump System, That Is) Our system, in a slide-show of operating statuses throughput a typical year. For each period typical temperatures are given and the ‘typical’ direction of heat flow.

System in September - typical operations conditions[2016-01-22] Temperature Waves and Geothermal Energy. ‘Geothermal’ energy used by heat pumps is mainly stored solar energy. A simple model: The temperature at the surface of the earth varies sinusoidally throughout the year – this the boundary condition for the heat equation. This differential equation links the temporal change of temperature to its spatial variation. I solve the equation, show some figures, and check how results compare to the thermal diffusivity of ground obtained from measurements.

Measured 'wave' and propagation time[2016-03-01] Rowboats, Laser Pulses, and Heat Energy (Boring Title: Dimensional Analysis). Re-visiting heat transport and heat diffusion length, this time only analyzing dimensional relationships. By looking at the heat equation (without the need to solve it) a characteristic length can be calculated: ‘How far does heat get in a certain time?’

Temperature waves in ground - attenuation length of about 10 meters[2017-02-05] Earth, Air, Water, and Ice. Data analysis of the heating season 2014/15 (when we turned off the solar/air collector to simulate a harsher winter) – and an attempt to show energy storages, heat exchangers, and heat flows in one schematic. From the net energy ‘in the tank’ the contribution of ground can be calculated.

Energy storage, heat exchangers, heat flow[2017-02-22] Ice Storage Hierarchy of Needs. Continued from the previous post – bird’s eye view: How much energy comes from which sources, and which input parameters are critical? I try to answer when and if simple energy accounting makes sense in comparison to detailed simulations.

Hierarchy of needs - ambient energy in ice months[2017-05-02] Simulating Peak Ice. I compare measurements of the level in the tank with simulations of the evolution of the volume of ice. Simulations (1-minute intervals) comprise a model of the control logic, the varying performance factor of the heat pump, heat transport in ground, energy balances for the hot and cold tanks, and the heat exchangers connected in series.

Simulations of brine and tank temperature and volume of ice, based on system state in 1-minute intervals.(Adding the following after having published this post. However, there is no guarantee I will update this post forever ;-))

[2017-08-17] Simulations: Levels of Consciousness. Bird’s Eye View: How does simulating heat transport fit into my big picture of simulating the heat pump system or buildings or heating systems in general? I consider it part of the ‘physics’ layer of a hierarchy of levels.

Simulation - levels of consciousness

Where to Find What?

I have confessed on this blog that I have Mr. Monk DVDs for a reason. We like to categorize, tag, painstakingly re-organize, and re-use. This is reflected in our Innovations in Agriculture …

The Seedbank: Left-over squared timber met the chopsaw.

The Nursery: Rebirth of copper tubes and newspapers.

… as well as in my periodical Raking The Virtual Zen Garden: Updating collections of web resources, especially those related to the heat pump system.

Here is a list of lists, sorted by increasing order of compactification:

But thanks to algorithms, we get helpful advice on presentation from social media platforms: Facebook, for example, encouraged me to tag products in the following photo, so here we go:

“Hand-crafted, artisanal, mobile nursery from recycled metal and wood, for holding biodegradable nursery pots.” Produced without crowd-funding and not submitted to contests concerned with The Intersection of Science, Art, and Innovation.

Ice Storage Hierarchy of Needs

Data Kraken – the tentacled tangled pieces of software for data analysis – has a secret theoretical sibling, an older one: Before we built our heat source from a cellar, I developed numerical simulations of the future heat pump system. Today this simulation tool comprises e.g. a model of our control system, real-live weather data, energy balances of all storage tanks, and a solution to the heat equation for the ground surrounding the water/ice tank.

I can model the change of the tank temperature and  ‘peak ice’ in a heating season. But the point of these simulations is rather to find out to which parameters the system’s performance reacts particularly sensitive: In a worst case scenario will the storage tank be large enough?

A seemingly fascinating aspect was how peak ice ‘reacts’ to input parameters: It is quite sensitive to the properties of ground and the solar/air collector. If you made either the ground or the collector just ‘a bit worse’, ice seems to grow out of proportion. Taking a step back I realized that I could have come to that conclusion using simple energy accounting instead of differential equations – once I had long-term data for the average energy harvesting power of the collector and ground. Caveat: The simple calculation only works if these estimates are reliable for a chosen system – and this depends e.g. on hydraulic design, control logic, the shape of the tank, and the heat transfer properties of ground and collector.

For the operations of the combined tank+collector source the critical months are the ice months Dec/Jan/Feb when air temperature does not allow harvesting all energy from air. Before and after that period, the solar/air collector is nearly the only source anyway. As I emphasized on this blog again and again, even during the ice months, the collector is still the main source and delivers most of the ambient energy the heat pump needs (if properly sized) in a typical winter. The rest has to come from energy stored in the ground surrounding the tank or from freezing water.

I am finally succumbing to trends of edutainment and storytelling in science communications – here is an infographic:

Ambient energy needed in Dec/Jan/Fec - approximate contributions of collector, ground, ice

(Add analogies to psychology here.)

Using some typical numbers, I am illustrating 4 scenarios in the figure below, for a  system with these parameters:

  • A cuboid tank of about 23 m3
  • Required ambient energy for the three ice months is ~7000kWh
    (about 9330kWh of heating energy at a performance factor of 4)
  • ‘Standard’ scenario: The collector delivers 75% of the ambient energy, ground delivers about 18%.
  • Worse’ scenarios: Either collector or/and ground energy is reduced by 25% compared to the standard.

Contributions of the three sources add up to the total ambient energy needed – this is yet another way of combining different energies in one balance.

Contributions to ambient energy in ice months - scenarios.

Ambient energy needed by the heat pump in  Dec+Jan+Feb,  as delivered by the three different sources. Latent ‘ice’ energy is also translated to the percentage of water in the tank that would be frozen.

Neither collector nor ground energy change much in relation to the base line. But latent energy has to fill in the gap: As the total collector energy is much higher than the total latent energy content of the tank, an increase in the gap is large in relation to the base ice energy.

If collector and ground would both ‘underdeliver’ by 25% the tank in this scenario would be frozen completely instead of only 23%.

The ice energy is just the peak of the total ambient energy iceberg.

You could call this system an air-geothermal-ice heat pump then!


Continued: Here are some details on simulations.

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. 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.

How Does It Work? (The Heat Pump System, That Is)

Over the holidays I stayed away from social media, read quantum physics textbooks instead, and The Chief Engineer and I mulled over the fundamental questions of life, the universe and everything. Such as: How to explain our heat pump system?

An astute reader suggested to create an ‘animation’ of the gradual evolution of the system’s state. As I learned from discussions, one major confusion was related to the role of the solar collector and the fact that you have to factor in the history of the heat source: This is true for every heat pump system that uses a heat source that can be ‘depleted’, in contrast to a flow of ground water at a constant temperature for example.

I don’t want to explain the thermodynamic details, and I think that the refrigerator analogy is not helpful. So for this pragmatic introduction a heat pump is just a device that generates heating energy as an output, the input energy being electrical energy and heat energy extracted from a rather cold heat source somewhere near the building. For 8kW heating power you need about 2kW electrical energy and 6kW ambient energy. The ratio of 8kW and 2kW is called the coefficient of performance.

What the typical intro to heat pumps in physics textbooks does not point out is that the ambient heat source actually has to be able to deliver that input energyduring a whole heating season. There is no such thing as the infinite reservoir of energy usually depicted as a large box. Actually, the worse the performance of a heat pump is – the ratio of output heat energy and input electrical energy, the smaller are the demands on the heat source. The Chief Engineer has coined the term The Heat Source Paradox for this!

The lower the temperature of the heat source, the smaller the coefficient of performance is: So if you run an air source heat pump in mid-winter (using a big ventilator) then less energy is extracted from that air source than a geothermal heat pump would extract from ground. But if you build a geothermal heat source that’s too small in relation to a building’s heating demands, you see the same effect: Ground freezes, source temperature decreases, performance decreases, and you need more electrical energy and less ambient energy.

I am harping on the role of the heat source as the whole point of our ‘innovation’ is our special heat source that has two components, both of them being essential: An unglazed solar / air collector and an underground water / ice tank plus the surrounding ground. The collector allows to replenish the energy stored in the tank quickly, even in winter: Air temperature just needs to be some degrees warmer than the cold brine. The tank is a buffer: When no energy is harvested by the collector at ambient temperatures below 0°C, water freezes and releases latent heat. So you can call that an air heat pump with a huge, silent and mainentance-free ‘absorber’ plus a buffer that provides energy for periods of frost and that allows for storing all the energy you don’t need immediately. Ground does provide some energy as well, and I am planning to post about my related simulations.(*) It can be visualized as an extension of the ice / water energy storage into the surroundings. But the active volume or area of ground is smaller than for geothermal systems as most of the ambient energy actually comes from the solar / air collector: The critical months in our climate are Dec-Jan-Feb: Before and after, the collector would be sufficient as the only heat source. In the three ‘ice months’ water is typically frozen in the tank, but even then the collector provides for 75-80% of the ambient energy needed to drive the heat pump.(*)

(*) Edit: This post written in 2017 shows how much energy is stored / exchanged by each component. An overview of essential numbers is given here; emphasis on the volume of ice – which is compared to simulations here.

Components are off-the-shelf products, actually rather simple and cheap ones, such as the most stupid, non-smart brine-water heat pump. What is special is 1) the arrangement of the heat exchanger pipes in the water tank and 2) the custom control logic, that is programming of the control unit.

So here is finally the series of images of the system’s state, shown in a gallery and with captions: You can scroll down to see the series embedded in the post, or click on the first image to see an enlarged view and then click through the slide-show.

More information on the system (technical data, sizing) and measurement data since 2012 can be found in this documentation.

German readers: This post contains the German version of this slide-show.

Economics of the Solar Air Collector

In the previous post I gave an overview of our recently compiled data for the heat pump system.

The figure below, showing the seasonal performance factor and daily energy balances, gave rise to an interesting question:

In February the solar collector was off for research purposes, and the performance factor was just a bit lower than in January. Does the small increase in performance – and the related modest decrease in costs of electrical energy – justify the investment of installing a solar/air collector?

Monthly Performance Factor, Heat Pump System

Monthly heating energy provided by the heat pump – total of both space heating and hot water, related electrical input energy, and the ratio = monthly performance factor. The SPF is in kWh/kWh.

Daily energy balances, heat pump system, season 2014-2015

Daily energies: 1) Heating energy delivered by the heat pump. Heating energy = electrical energy + ambient energy from the tank. 2) Energy supplied by the collector to the water tank, turned off during the Ice Storage Challenge. Negative collector energies indicate cooling of the water tank by the collector during summer nights. 200 kWh peak in January: due to the warm winter storm ‘Felix’.

Depending on desired pay-back time, it might not – but this is the ‘wrong question’ to ask. Without the solar collector, the performance factor would not have been higher than 4 before it was turned off; so you must not compare just these two months without taking into account the history of energy storage in the whole season.

Bringing up the schematic again; the components active in space heating mode plus collector are highlighted:

Space heating with solar collector on, heat pump system punktwissen.

(1) Off-the-shelf heat pump. (2) Energy-efficient brine pump. (3) Underground water tank, can also be used as a cistern. (4) Ribbed pipe unglazed solar collector (5) 3-way valve: Diverting brine to flow through the collector, depending on ambient temperature. (6) Hot water is heated indirectly using a large heat exchanger in the tank. (7) Buffer tank with a heat exchanger for cooling. (8) Heating circuit pump and mixer, for controlling the supply temperature. (9) 3-way valve for switching to cooling mode. (10) 3-way valve for toggling between room heating and hot water heating.

The combination of solar collector and tank is ‘the heat source’, but the primary energy source is ambient air. The unglazed collector allows for extracting energy from it efficiently. Without the tank this system would resemble an air heat pump system – albeit with a quiet heat exchanger instead of a ventilator. You would need the emergency heating element much more often in a typical middle European winter, resulting in a lower seasonal performance factor. We built this system also because it is more economical than a noisy and higher-maintenance air heat pump system in the long run.

Our measurements over three years show that about 75%-80% of the energy extracted from the tank by the heat pump is delivered to it by the solar collector in the same period (see section ‘Ambient Energy’ in monthly and yearly overviews). The remaining energy is from surrounding ground or freezing water. The water tank is a buffer for periods of a few very cold days or weeks. So the solar/air collector is an essential component – not an option.

In Oct, Nov, and March typically all the energy needed for heating is harvested by the solar collector in the same month. In ‘Ice Months’  Dec, Jan, Feb freezing of water provides for the difference. The ice cube is melted again in the remaining months, by the surplus of solar / air energy – in summer delivered indirectly via ground.

The winter 2014/2015 had been unusually mild, so we had hardly created any ice before February. The collector had managed to replenish the energy quickly, even in December and January. The plot of daily energies over time show that the energy harvested by the collector in these months is only a bit lower than the heating energy consumed by the house! So the energy in the tank was filled to the brim before we turned the collector off on February 1. Had the winter been harsher we might have had 10 m3 of ice already on that day, and we might have needed 140kWh per day of heating energy, rather than 75kWh. We would have encountered  the phenomena noted during the Ice Storage Challenge earlier.

This post has been written by Elke Stangl, on her blog. Just adding this in case the post gets stolen in its entirety again, as it happened to other posts tagged with ‘Solar’ recently.

Solar Energy, Batteries, and Autonomy

This is the third post in my series on our photovoltaic generator. It had been a part of previous post with the data for the first month, but I cut and saved it as the other post was so long already.

I am now also able to present data for two months of operations: Below is an updated plot of daily energy balances in the second month. Again, I am combining data from our photovoltaic inverter’s logging and from our meters that track the difference of consumption and fed-in energy.

There are two ways to judge the system’s performance in relation to your ‘autonomy’ and economics:

  • Self-sufficiency quota: Which part of energy consumption in the house is harvested from the solar panels?
  • Self-consumption quota: Which percentage of the PV power created can be  consumed immediately.

Energy from PV = data from our inverter’s data logger. Energy from / fed-into grid: Calculated from data logged by our smart meter(s). Arrows indicate which axis to use. By mid of June, our own smart meter had been installed so we did not have to read off the dumb smart meter’s display manually any more.

June was sunny and hot: We harvested more energy due to the higher number of sunny days, but we did not reach May’s daily high score of 32,88 kWh again: Longer days cannot compensate for the reduction of the efficiency of the panels due to higher ambient air temperatures (as discussed in the previous post).

Comparing May and June I noticed:

  • We need less energy per day on average: 10 kWh instead of 11 kWh. Either we spend more time outside than sitting in front of a computer, need less light, or (most likely) we are too obsessed with playing with our new metering gadgets and optimizing our energy consumption.
  • As a consequence – especially in combination with a higher average PV energy output of 25,5 kWh versus 22,1 kWh – our self-consumption quota slightly decreased: We could use only 25,8% of PV power directly, versus 27,5% in the month before.
  • For the same reason, we are more ‘autonomous’, self-sufficiency quota increased: On average 66% of the energy needed in the house was directly delivered by the solar panels (Before: 55%).

Now the tantalizing questions are:

Which quotas will be reach for a full year? How would those numbers change if you had a battery? Would this justify the investment?

The following is my personal assessment, based on our (Austrian) market of electricity and our way of using electrical energy as home owners.

My assessment is based on the following:

  • I pay three times more for electrical power than I will earn when feeding in to the grid (about € 0,18 versus € 0,06).
  • Thus the goal is to maximize the amount of energy used in house and only sell to the utility if you cannot use your power.
  • A battery can store the surplus of energy harvested during the day, so that you can consume it in the night or when clouds pass.
  • The ‘profit’ from having it battery is thus the difference in costs of power and feed-in tariffs (€ 0,12), multiplied by the energy you are able to ‘shift’ to the darker hours.

To determined estimated profits I need to compare our self-sufficiency or self-consumption quotas with and without a battery. In June 2016 I will have data for the latter – energy generation and consumption for every 15-minute time slot. A battery can then be added in a simulation using this logic:

  • Energy not consumed in the house is used for charging the battery unless it if fully charged already.
  • If the solar power is not sufficient to cover current demands, energy is delivered by the battery – unless it has reached its minimum allowed capacity (80% of nominal capacity for Li-in batteries, 50% for lead-acid ones).
  • The conversion of energy is not perfect and battery losses need to be accounted for.

Before I will be able to do this, I play with ballpark numbers, see e.g. inverter vendor SMA’s planning guide (diagrams and examples p.32), or somebody else’s load profile. This tool by a German developer allows for playing with the size of the battery and PV generator interactively (the legend of the plot is in German, but the monthly balances and input fields are titled in English).

Those tools and diagrams need the following input data:

  • Yearly energy consumption: About 7500 kWh in our case, more than 4000 kWh is for the heat pump.
  • Yearly energy harvest from solar panel: Using one of the free PV simulation tools that take into account weather data (e.g. PVGIS provided by the European union) we should end up with 5300 kWh.
  • Size of the battery and percentage of usable capacity. I play with a hypothetical Li-ion battery with 10 kWh, thus 8 kWh maximum energy.

At the German website you can pick different load profiles, I compare different ones that might be similar to our usage pattern.

The results indicate that …

  • the estimated self-sufficiency quota for a full year would be about 30% for our system; and it would increase from about 30% to 55% on adding a battery.
  • we might be able to shift less than 2000 kWh per year by the battery. Note that calculations based on self-consumption quota also include the battery losses – about 500 kWh per year – so data based on the self-sufficiency quota should be used to see the real usable energy.

Multiplying this with the difference in costs and profits (€ 0,12 per kWh) would result in a profit added by the battery of about € 240 per year. BTW this is less than 50% of the profit gained by using the PV generator without the battery.

I researched prices of Li-ion batteries before we purchased our generator and the cheapest one I found was a 8 kWh battery (nominal capacity) for € 6500. Tesla has recently announced its Li-ion Powerwall home battery with 10 kWh, to be sold to $ 3500 to contractors. Prices for installations and price difference between a ‘classical’ inverter and one that can manage a battery not included (less than € 500 here). If you have the battery installed together with the panels, there should not be much additional installation efforts though, compared to the time needed for working on the roof. But I think a 10 kWh battery will not be cheaper than something like € 4000 unless one more ‘disruption’ hits the industry.

So the estimated profits of € 240 are considerably less than 10% of the costs of the battery. Since the estimated life-time of the battery is 10-20 years it is likely not to pay itself off until it has to be replaced again. (No interest rates and realistic net present value considered.) The solar panels will last for decades and still deliver power – though at a slowly deteriorating efficiency – when the payback period has passed.

Those 2000 kWh might even have been too optimistic: The more energy you can use immediately, the less economic is the battery – as there is less energy left to be shifted at all:

  • We use the office during the day: I had always noticed that electricity bills from the past clearly show if we were commuting to a workplace far away or using our own office.
  • We use a heat pump, and we schedule the hot water cycle to match the sunny hours.

I think that you should try to use energy more efficiently first – before adding a battery to shift unnecessary loads. The decision for a battery should be based on an optimized load profile.

For example, Tesla presents some data for a typical energy consumption of devices on their Powerwall website – see bottom of this page. A refrigerator should use 4,8 kWh a day? If I’d have such a refrigerator I would rather replace this than investing into a battery. A modern fridge should not use more more than a kWh per day.

Using a heat pump in middle Europe I might have an unusual view of autonomy. But I believe heating energy in northern latitudes is often neglected in discussions about becoming more self-sufficient by using solar and wind power. If you don’t want to depend on a utility, why would you want to depend on the vendors of fossil fuel or wood – and related financial markets and volatile prices? Here, we have a very reliable power infrastructure, access to 100% green power harvested locally (wind or hydro). But oil and gas have to be imported, and the energy for heating in winter is several times the electrical energy consumed by appliances.

A heat pump the best way to utilize solar and wind power for heating, but you have to face solar energy’s obvious disadvantage: You cannot store summer’s surplus of PV energy for winter (not until a commercially feasible combined fuel cell plus electrolyzer will be available). PV energy from generators that fit onto typical roofs are too small to cover heating energies in winter. Our expected PV output in the coldest months will be less than 10 kWh per day, whereas the heat pump might need up to 35 kWh (one quarter of heating energy). And lest we not forget power versus energy: The heat pump always operates at full power, with its rated power being determined by a worst case minimum ambient temperature. You will need to input something like 2-3 kW for a rather short period. The more average power you need, the more often the heat pump is turned on. So you also need 2-3 kW of solar power or battery output power – a challenge given irradiance in winter and typical battery inverter’s output powers.

In summer a 10 kWh battery the capacity of Tesla’s will be fully charged quickly: You can expect half a year of autonomy, but have to feed in your surplus to the grid. In winter, the battery will hardly be charged as all power is consumed immediately. So the estimated self-sufficiency quota will be well below the absolute theoretical maximum of yearly PV energy harvest over yearly energy consumption (about 70% in our case).

Perhaps you wonder why I am only pontificating on the economics of batteries. I haven’t mentioned the battery’s function as a backup system yet for these reasons:

  • Expected downtime per utility client here is less than an hour per year.
  • In winter a backup system would certainly be more interesting if you depend on a heat pump – but, as mentioned above, the battery will hardly every be charged. The storage  and latency built in in heat pump systems will cover the typical downtimes easily. You use a storage of heat rather than one of electrical energy.
  • You might need to plan carefully which devices are allowed to run during a blackout, as the output power of inverters is limited, and typically of the same order as an electric stove or a water cooker (2 – 3 kW – as high as the heat pump’s input power!). If I only want to be sure that my computer does not crash or want to have a chance to look up the utility’s website, then I’d rather go for a UPS (Uninterruptable Power Supply). A UPS is a battery you connect your important devices to directly, whereas a battery-powered backup system plus inverter needs to maintain a small AC network, controlling frequency and voltage like The Big Grid.
  • A full-featured backup system compliant with our local safety regulations will need some components not included in vendors’ turn-key system. At least a few months ago it was hard to obtain a definitive and unambiguous statement about what exactly the full backup solution would entail.

Another thing I am still waiting for is the option to use a car battery (easily) also as a home storage battery. AFAIK car batteries as the Tesla’s do not allow for bidirectional charging and consumption. (Edit Dec. 2015: Nissan’s LEAF can do that but this solution had not been available in Europe yet).

But fossil fuel burnt by cars are much tougher to get rid of in Austria than fossil fuel burnt in homes. Statistics show it is seems very hard to reduce the former, whereas the latter is gradually decreasing as people prefer heat pumps or pellet stoves today.

I have reduced my carbon footprint drastically in recent years: by driving much less and turning to ‘remote work’. Now we would be ready to replace one of our two cars by an electrical one, as we can plan better when and how it will be used. But why buy the battery twice – one for the car that seldom uses it and one for the home?

That said, one should always keep in mind seemingly unrelated investments and not apply double standards. For example, a lightning protection system for a house like ours costs about € 4000. So if I pay that for mitigating an event due every 1000 years (estimated by a tool provided by our national weather agencies – probability of a lightning strike) – I sort of nearly feel obliged or entitled to buy a battery.

I am also aware of the fragility, if not absurdity, of financial forecasts in times like this – I don’t even need to factor in the collapse of the eurozone or something. I have just heard rumours about drastic changes to be made to utilities’ pricing models. Utilities here start to lose money when serving PV system owners: In those € 0,18 per kWh fees for transmission via the grid are included. But if you deliver energy to the grid you are not charged for kWh transmitted. Transmission system operators have to swallow your energy and are burdened with managing the volatility of renewable energy. Currently running costs of electricity are high whereas the fixed costs like metering (which I factored in in that € 0,18) are negligible. An envisaged model might comprise a rather high base fee instead – no matter if you are a consumer and/or run a power plant – plus low running costs of about € 0,10 / kWh. If those rumours come true, it would become more interesting to go off-grid completely which is not an option given the constraints I outlined above (summer/winter, heat pump, high peak power of appliances).