Alien Energy

I am sure it protects us not only from lightning but also from alien attacks and EMP guns …

So I wrote about our lightning protection, installed together with our photovoltaic generator. Now our PV generator is operational for 11 months and we have encountered one alien attack, albeit by beneficial aliens.

The Sunny Baseline

This is the electrical output power of our generator – oriented partly south-east, partly south-west – for some selected nearly perfectly cloudless days last year. Even in darkest winter you could fit the 2kW peak that a water cooker or heat pump needs under the curve at noon. We can heat hot water once a day on a really sunny day but not provide enough energy for room heating (monthly statistics here).

PV power over time: Sunny days 2015

Alien Spikes and an Extended Alien Attack

I was intrigued by very high and narrow spikes of output power immediately after clouds had passed by:

PV power over time, data points taken every few seconds.

There are two possible explanations: 1) Increase in solar cell efficiency as the panels cool off while shadowed or 2) ‘focusing’ (refraction) of radiation by the edges of nearby clouds.

Such 4kW peaks lasting only a few seconds wide are not uncommon, but typically they do not show up in our standard logging, comprising 5-minute averages.

There was one notable exception this February: Power surged to more than 4kW which is significantly higher than the output on other sunny days in February. Actually, it was higher than the output on the best ever sunny day last May 11 and as high as the peaks on summer solstice (Aliens are green, of course):

PV power over time: Alien Energy on Feb 11, 2016

Temperature effect and/or ‘focusing’?

On the alien attack day it was cloudy and warmer in the night than on the sunny reference day, February 6. At about 11:30 the sun was breaking through the clouds, hitting rather cool panels:

PV power over time: February 2016 - Output Power and Ambient Temperature

At that day, the sun was lingering right at the edge of clouds for some time, and global radiation was likely to be higher than expected due to the focusing effect.

Global Radiation over time: February 2016

The jump in global radiation at 11:30 is clearly visible in our measurements of radiation. But in addition panels had been heated up before by the peak in solar radiation and air temperature had risen, too. So the different effects cannot be disentangled easily .

Power drops by 0,44% of the rated power per decrease in °C of panel temperature. Our generator has 4,77kW, so power decreases by 21W/°C panel temperature.

At 11:30 power was by 1,3kW higher than power on the normal reference day – the theoretical equivalent of a panel temperature decrease by 62°C. I think I can safely attribute the initial surge in output power to the unusual peak in global radiation only.

At 12:30 output power is lower by 300W on the normal sunny day compared to the alien day. This can partly be attributed to the lower input radiation, and partly to a higher ambient temperature.

But only if input radiation is changing slowly, panel temperature has a simple, linear relationship with input temperature. The sun might be blocked for a very short period – shorter than our standard logging interval of 90s for radiation – and the surface of panels cools off intermittently. It is an interesting optimization problem: By just the right combination of blocking period and sunny period overall output could be maximized.

Re-visiting data from last hot August to add more dubious numbers

Panels’ performance was lower for higher ambient air temperatures …

PV power over time: August 2015 - Output Power and Ambient Temperature

… while global radiation over time was about the same. Actually the enveloping curve was the same, and there were even negative spikes at noon despite the better PV performance:

Global Radiation over time: August 2015

The difference in peak power was about 750W. The panel temperature difference to account for that would have to be about 36°. This is three times the measured difference in ambient temperature of 39°C – 27°C = 12°C. Is this plausible?

PV planners use a worst-case panel temperature of 75°C – for worst-case hot days like August 12, 2015.

Normal Operating Cell Temperature of panels is about 46°C. Normal conditions are: 20°C of ambient air, 800W/m2 solar radiation, and free-standing panels. One panel has an area of about 1,61m2; our generator with 18 panels has 29m2, so 800W/m2 translates to 23kW. Since the efficiency of solar panels is about 16%, 23kW of input generates about 3,7kW output power – about the average of the peak values of the two days in August. Our panels are attached to the roof and not free-standing – which is expected to result in a temperature increase of 10°C.

So we had been close to normal conditions at noon radiation-wise, and if we had been able to crank ambient temperature down to 20°C in August, panel temperature had been about 46°C + 10°C = 56°C.

I am boldly interpolating now, in order to estimate panel temperature on the ‘colder’ day in August:

Air Temperature Panel Temperature Comment
20°C 56°C Normal operating conditions, plus typical temperature increase for well-vented rooftop panels.
27°C 63°C August 1. Measured ambient temperature, solar cell temperature interpolated.
39°C 75°C August 12. Measured ambient temperature.
Panel temperature is an estimate for the worst case.

Under perfectly stable conditions panel temperature would have differed by 12°C, resulting in a difference of only ~ 250W (12°C * 21W/°C).

Even considering higher panel temperatures at the hotter day or a non-linear relationship between air temperature and panel temperature will not easily give you the 35° of temperature difference required to explain the observed difference of 750W.

I think we see aliens at work again:

At about 10:45 global radiation for the cooler day, August 1, starts to fluctuate – most likely even more wildly than we see with the 90s interval. Before 10:45, the difference in output power for the two days is actually more like 200-300W – so in line with my haphazard estimate for steady-state conditions.

Then at noon the ‘focusing’ effect could have kicked in, and panel surface temperature might haved fluctuated between 27°C air temperature minimum and the estimated 63°C. Both of these effects could result in the required additional increase of a few 100W.

Since ‘focusing’ is actually refraction by particles in the thinned out edges of clouds, I wonder if the effect could also be caused by barely visible variations of the density of mist in the sky as I remember the hot period in August 2015 as sweltry and a bit hazy, rather than partly cloudy.

I think it is likely that both beneficial effects – temperature and ‘focusing’ – will always be observed in unison. On February 11 I had the chance to see the effect of focusing only (or traces of an alien spaceship that just exited a worm-hole) for about half an hour.

Wormhole travel as envisioned by Les Bossinas for NASA________________________________

Further reading:

On temperature dependence of PV output power:

On the ‘focusing’ effect:

  • Can You Get More than 100% Solar Energy?
    Note especially this comment – describing refraction, and pointing out that refraction of light can ‘focus’ light that would otherwise have been scattered back into space. This commentator also proposes different mechanism for short spikes in power and increase of power during extended periods (such as I observed on February 11).
  • Edge-of-Cloud Effect

Source for the 10°C higher temperature of rooftop panels versus free-standing ones: German link, p.3: Ambient air + 20°C versus air + 30°C

Having Survived the Hottest July Ever (Thanks, Natural Cooling!)

July 2015 was the hottest July ever since meteorological data had been recorded in Austria (since 248 years). We had more than 38°C ambient air temperature at some days; so finally a chance to stress-test our heat pump system’s cooling option.

Heating versus cooling mode

In space heating ‘winter’ mode, the heat pump extracts heat from the heat source – a combination of underground water / ice tank and unglazed solar collector – and heats the bulk volume of the buffer storage tank. We have two heating circuits exchanging heat with this tank – one for the classical old radiators in ground floor, and one for the floor heating loops in the first floor – our repurposed attic.

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

Space heating mode: The heat pump (1) heats the buffer tank (7), which in turn heats the heating circuits (only one circuit shown, each has its circuit pump and mixer control). Heat source: Solar/air collector (4) and water / ice storage (3) connected in a single brine circuit. The heat exchanger in the tank is built from the same ribbed pipes as the solar collector. If the ambient temperature is too low too allow for harvesting of energy the 3-way valve (5) makes the brine flow bypass the collector.

The heat pump either heats the buffer tank for space heating, or the hygienic tank for hot tap water. (This posting has a plot with heating power versus time for both modes).

We heat hot tap water indirectly, using a hygienic storage tank with a large internal heat exchanger. Therefore we don’t need to fight legionella by heating to high temperatures, and we only need to heat the bulk volume of the tank to 50°C – which keeps the Coefficient of Performance high.

Heating hot water, solar collector off, heat pump system punktwissen

Hot tap water heating mode: The flow of water heated by the heat pump is diverted to the hygienic storage tank (6). Otherwise, the heat source is used in the same way as for space heating. In this picture, the collector is ‘turned off’ – corresponding to heating water on e.g. a very cold winter evening.

In summer, the still rather cold underground water tank can be used for cooling. Our floor heating loops become cooling loops and we simply use the cool water or ice in the underground tank for natural (‘passive’) cooling. So the heat pump can keep heating water – this is different from systems that turn an air-air heat pump into an air conditioner by reverting the cycle of the refrigerant.

Heating hot water in parallel to cooling is beneficial as the heat pump extracts heat from the underground tank and cools it further!

Space cooling while heating hot water, heat pump system punktwissen

Cooling mode: Via automated 3-way valve (9) brine is diverted to flow through the heat exchanger in the buffer tank (7). Water in the buffer tank is cooled down so water in the floor ‘heating’ / cooling loops. If the heat pump operates in parallel to heat hot tap water, it cools the brine.

How we optimize cooling power this summer

Water tank temperature. You could tweak the control to keep the large ice cube as long as possible, but there is a the trade-off: The cooler the tank,  the lower the heat pump’s performance factor in heating mode. This year we kept the tank at 8°C after ‘ice season’ as long as possible. To achieve this, the solar collector is bypassed if ambient temperature is ‘too high’. The temperature in the tank rose quickly in April – so our ice is long melted:

Temperatures and performance factors, July 2015

The red arrow indicates the end of the ice period; then the set temperature of the tank was 8°C (‘Ice storage tank’ is rather a common term denoting this type of heat source than indicating that it really contains ice all the time.) Green arrows indicate three spells of hot weather. The tank’s temperature increased gradually, being heating by the surrounding ground and by space cooling. At the beginning of August its temperature is close to 20°C, so cooling energy has nearly be used up completely.

At the beginning of July the minimum inlet temperature in the floor loops was 17°C, determined by the dew point (monitored by our control system that controls the mixer accordingly); at the end of the month maximum daily ambient air temperatures were greater than 35°C, and the cooling water had about 21°C.

Room temperature. Cooling was activated only if the room temperature in the 1st floor was higher than 24°C – this allows for keeping as much cooling energy as possible for the really hot periods. We feel that 25°C in the office is absolutely OK as temperatures outside are more then 10°C higher.

Scheduling hot water heating. After the installation of our PV panels we set the hot water heating time slots to periods with high solar radiation – when you have more than 2 kW output power on cloudless days. So we utilized the solar energy generator in the most economic way and the heat pump supports cooling exactly when cooling is needed.

Using the collector for cooling in the night. If the ambient temperature drops to a value lower than the tank temperature, the solar collector can actually cool the tank!

Ventilation. I have been asked if we have forced ventilation, ductwork, and automated awnings etc. No, we haven’t – we just open all the windows during the night and ‘manually operated’ shades attached to the outside of the windows. We call them the Deflector Shields:

Ventilation: Night

Manually operated ventilation – to be shut off at sunrise. We had already 30°C air temperature at 08:00 AM on some days.

Deflector shields: Day

South-east deflector shields down. We feel there is still enough light in the (single large) room as we only activate the subset of shields facing the sun directly.

These are details for two typical hot days in July:

Temperature and cooling power for two days in July

The blue line exhibits the cooling power measured for the brine ‘cooling’ circuit. If the heat pump is off, cooling power is about 1 kW; during heat pump operations (blue arrows) 4 kW cooling power can be obtained. Night-time ventilation is crucial to keep room temperatures at reasonable levels.

The cooling power is lower than so-called standard cooling load as defined in AC standards – the power required to keep the temperature at about 24°C in steady-state conditions, when ambient temperature would be 30°C and no shades are used. For our attic-office this standard cooling power would amount to more than 10 kW which is higher than the standard (worst case) heating load in winter.

Overall electrical energy balance

I have been asked for a comparison of the energy needed in the house, the heat pump in particular, and the energy delivered by the PV panels and fed in to the grid.

PV numbers in July were not much different from June’s – here is the overview on June and July, maximum PV power on cloudless days has decreased further due to the higher temperatures:

Daily energy balance, PV generation and self-consumption-2015-06-and-2015-07In July, our daily consumption slightly decreased to 9-10 kWh per day, the heat pump needs 1-2 kWh of that. The generator provides for 23 kWh per day,

Currently the weather forecast says, we will have more than 35°C each noon and 20-25° minimum in the night until end of this week. We might experience the utter depletion of our cooling energy storage before it will be replenished again on a rainy next weekend.

More Ice? Exploring Spacetime of Climate and Weather.

I have become obsessed with comparing climate data for different regions in the world and in different years (space + time).

Finally I have found the tool I was looking for; now I can compare average Ice Days quickly – days with a maximum temperature < 0°C. In the first quarter of 2014 there were:

5 Ice Days in Vienna

compared to

68 Ice days Days in the Canadian prairie, in Regina, Saskatchewan, Canada.

It seems that a typical winter in Regina is about as grim as the winter in Europe in 1962/63, a 250 year event that has its own Wikipedia entry (DE version for Europe, EN article for UK). In this winter temperature was below 0°C for up to 120 days even in lowland areas. It was the most persistent cold since 1739. In Canada and Greenland temperatures were unusually mild in that season:

GHCN GISS HR2SST 1200km Anom1203 1963 1963 1949 1978

I am interesting in the probability of extremely cold days in a row as this determines the size of the water tank to be used a heat source. The Austrian national weather service provides data since 1994. I used the daily average ambient temperatures as an input for a crude simulation – to determine the maximum volume of ice in the tank per year. So I did accounting of the energy in water tank:

  • How much energy is needed for space heating and hot water, based on ambient temperature and number of persons?
  • For a constant performance factor of 4 the heat extracted by the heat pump from the tank is 3/4 of this. The assumption is reasonable as long as the tank is big enough which means the tank temperature will not be lower than 0°C.
  • How much energy can be gained from ambient air? Air temperature needs to be some degrees higher than brine temperature which has typically less than 0°C in winter.
  • How big is the contribution of ground? If the tank would be fully frozen only this contribution would matter – then the system had turned into a geothermal one.

This was a much simpler exercise than detailed simulations I did for selected seasons before – based on and data taken every hour or even every 3 minutes. In the daily accounting approach, I did not take into account the detailed hydraulic schema, every switching of a valve or the temperatures ‘before’ and ‘after’ the heat exchangers in the tank and in the air. It also speeded up calculations to replace numerical simulations of the heat flow and the ‘temperature waves’ in the surrounding ground below the tank by simple estimates.

I compared the results to measured volume of ice for the past two seasons and to a detailed simulations for specific seasons. Since the maximum volumes of ice are approximately the same I consider the simple simulation good enough for providing an overview and some ‘feel’ about what different winters will result in.

Our tank is ~27m3 size in, thus allowing for ~25m3 of ice maximum as the volume is increased by 10% on freezing. It would have hit the limit in 1996 and 1997:

Volume of ice, water tank as a heat source of heat pump. Simulation for 1994-2013.

Volume of ice in the tank, determined by ambient temperature. The peak for 2005/2006 is in agreement with a ‘real’ detailed simulation, the peak for 2012/2013 with the measured value. The small peak for 2013 is still larger than the observed value as the simple simulation based on daily values only breaks down if the ‘lifetime’ of the ice peak is too small.

Every heat pump system has an option to switch to a heating element in case the heat source is exhausted. With air heat pumps, the ambient air simply gets too cold. Geothermal systems utilize a big volume of soil, so the source would be exhausted just as our tank when a large volume of soil is frozen. Limiting factors are the freezing point of brine and the thermodynamic properties of the refrigerant.

We would have used the heating element 2 times in 20 years for a few days. This could be prevented if the tank was built bigger; finally it boils down to an economic assessment:

  • Our house needs about 20.000 kWh per year of energy (hot water included) This is a conservative estimate – in the season 2013/14 we needed only 17.500 kWh.
  • As long as the tank is not frozen, the performance factor is 4 and thus 3/4 of this will be provided by ‘the environment’ / the tank: 15.000 kWh.
  • The remaining energy is the electrical energy consumed by the heat pump’s compressor: 5.000 kWh.
  • On a very cold day the heating energy is: 130 kWh (equivalent to 5,4 kW – so still below design heating load); about 98 kWh are extracted from the tank.
  • The tank contains about 2.000 kWh latent heat and can sustain about 20 very cold days.
  • The ‘ten year colds’ lasted for a few days more. Four more days would require about 500 kWh extra, by 1:1 heating.

I highlighted the essential numbers to be compared: Once in ten years, electrical heating energy would be higher by about 10%. On average (per year), this would add only  1%. 1% of the yearly utility bill’s total need to be compared to the costs of building a larger tank.

In an exceptional winter like season 1962/63 about two more months had to be sustained: Heating at worst case power for 60 days is equivalent to 7.800 kWh; and using a 1:1 heating element means an excess electrical energy of 5.850 kWh – those 3/4 of the total heating energy that would otherwise (before the tank is exhausted) be provided by ‘the environment’. This has to be compared to standard consumption for 250 years, that is: 250 times 5.000 = 1.250.000 kWh. Thus excess heating energy amounts to less than 0,5%.

One might argue that 250 years does not make sense as you might at best consider heating costs for one human being’s life time – and you might encounter such a season, or not. But after all, these numbers would just provide some way of comparing different heating systems – all of which would result in excessive heating costs in such a winter no matter what the fuel was.