Solar power supply up to 75% grid independent in the center of germany for private and industrial consumers via storage accumulators

Not only for roofs oriented south but also for roofs oriented east and west–

ANTARIS AS M 190: The PV module with the highest energy yield

Author: Christoph Geißler, B.Eng. Mechatronics
Co-Author: Eberhard Zentgraf, Certified Electronic Engineer

Scientific team involved in designing, planning, installing, measuring, recording and evaluating:
C. Geißler
E. Zentgraf
A. Wolf
S. Hock
A. Zentgraf
S. Schulz
T. Staab
M. Moore

A. list of Contents

A. List of Contents 3
B. List of Figures 4
C. List of Tables 5
1. Preface 7
2. Yields of a south-oriented roof 8
3. Yield of east- plus west oriented roof 11
4. Comparison of south versus east-west 13
4.1 Comparison of yields 13
4.2 Economical view without storage media 14
4.3 Economical view with storage media 16
5. Optimizing own consumption without storage 19
5.1 Self initiative 19
5.2 Smart Metering 19
6. Island systems with possible highest autarchy 20
6.1 Real measuring to autarchy 21
7. Own consumption with electrochemical batteries 25
7.1. Conventional battery technologies 25
7.1.1 Lead-acid batteries 25
7.1.2 Redox-Flow 26
7.2. More accumulation technologies 27
7.2.1 Li-ion technology 27
7.2.2 Nickel battery 28
8. Optimization of own consumption with other storage media 29
8.1. Hydrogen 29
8.2. Methane 29
8.3. Methanol 29
8.4. Compressed air 29
8.5. Flywheel energy storage 30
8.6. Pumped hydro storage 30
8.7. Water heating 31
8.8. Refrigerators / Freezers 31
8.9. Air conditioning units / air conditioning systems 31
8.10. Ice-Heating 31
8.11. CHP (Cogeneration / combined
9. Conclusion and comments to an east-west oriented roof layout and to the optimization of own-consumption 34

D. Index of web sources 35

B. List of Figures

Fig 1: Percentage of highest possible yield depending upon orientation and angle of inclination of the roof [1]. 8
Fig 2:Performance curve of panels in south orientation standardized to one kWp during sunshine on May 26th, 2012 9
Fig 3:Performance curve of panels in south orientation standardized to one kWp during sunshine on May 26th, 2012 9
Fig 4: Performance curve of south oriented panels standardized to one kWp during considerably cloudy skies on May 15th, 2012. 10
Fig 5: Global irradiation on May 15th, 2012 10
Fig 6:Performance curve of panels in east-west-alignment standardized to one kWp during sunshine. 12
Fig 7: Performance curve of panels in east-west-orientation standardized to one kWp during considerably cloudy skies. 12
Fig 8: Comparison of yields of south versus east-west on May 26th, 2012 13
Fig 9: Comparison of yields south versus east-west on May 15th, 2012 14
Fig 10: Yields of a south oriented roof (5 kWp) and an east (3 kWp) plus west (3 kWp) oriented roof in comparison to a daily demand 21
Fig 11: Yields of a south oriented system (5 kWp) and an east- (5 kWp) west (5 kWp) system in comparison to the daily demand 22
Fig 12: Excess and purchase of power with a south oriented system with 5 kWp 23
Fig 13: Excess and purchase of power with an east (3 kWp) plus west (3 kWP) system 23
Fig 14: Excess and purchase of power with an east (5 kWp) plus west (5 kWp) system. 24
Fig 15: Schema of lead-acid battery [2] 26
Fig 16: Schema of a Redox-Flow battery [2] 27
Fig 17: Schema of a Li-ion accumulator [3] 28
Fig 18: Schema of a pumped storage hydro power station [6] 31
Fig 19: Schema of an ice-heater [7] 32
Fig 20: Schema of a CHP [8] 33

C. List of Tables
Table 1: Base value for our south oriented system 14
Table 2: Balances with virtual energy price increments for south oriented PV system 15
Table 3: System values of an east-west oriented roof 15
Table 4: Balances for a 6 kWp system 15
Table 5: Balances for a 10 kWp east-west oriented system 16
Table 6: Base values for our south oriented system 16
Table 7: Balances with virtual energy price increments 17
Table 8: System values of an east-west system with energy storage 17
Table 9: Balance of 6 kWp east-west oriented system 17
Table 10: Balance of a 10 kWp east-west oriented system 18
Table 11: Overview of different systems 22

1. Preface

This review performed by the TEC institute for technical innovations (TEC Institut für technische Innovationen GmbH & Co. KG) deals with the optimization of photovoltaic own consumption and the so-called east-west roof occupancy with photovoltaic modules (panels). An east-west alignment demonstrates an alternative to the conventional roof occupancy facing south in which the panels are preferably installed. In this case, the panels are being installed towards east as well as west.

Based on our own measurements and research we are able to demonstrate that PV systems facing east and west require only approximately additional 20% panels to reach the almost exact yield as a system with a complete south orientation.

The coherency of this and the optimization of own consumption with and without storage is described in detail in the following chapters.

This subject matter evolved on the basis of permanently decreasing feed-in remunerations and inconstant politics the PV industrial sector does not wish to nor is able to depend upon any longer if they should remain competitive. Based on the current political situation and lobbyistic influences of the major electric power companies, it is to be expected that the feed-in remunerations in 2013 are no longer cost-effective and will be omitted on short term notice. However, the energy reference prize shall rise continuously. The following report shows, amongst others, that PV systems in fact are profitable if as much as possible of the self produced PV energy is being self consumed. As a conclusion of our research and measuring, we can display that 75% autonomy in the middle of Germany is indeed possible. It is therefore very important that everyone contributes to the energy turnaround to make Germany’s electricity generation more environmentally friendly.

2. Yields of a south-oriented roof

As reference to our east-west roof occupancy we use a south oriented roof. Standardizing takes place on one kWp. In the following diagrams we show a sunny day (fig 2) and a considerably cloudy day (fig 4). The data was simultaneously recorded with the data of the panels facing east and west.

Some of our panels are installed at an inclination angle of 30° and a 0° deviation facing south. This results in a yield of app. 100%, according to our table (fig 1). In reference to that we measured the global irradiation.

Percentage of highest possible yield depending on orientation and inclination angle of the roof [1]

Fig 1: Percentage of highest possible yield depending on orientation and inclination angle of the roof [1].

 

Case study:
In the following case study (installed system performance of 1kWP) a demand of 320W for as long as possible is to be established. We chose May 26th, 2012 as an example for such observation. Since the 1kWp system yielded a peak performance of about 800Wp on this day 40% of the reached peak value equates to 320W.

As displayed in fig 2 and fig 3, May 26th, 2012 was a nice and sunny day at large, except for some minor clouds around noon. We could also observe the curve progression of metered panels oriented south (fig 2) coincides with the curve progression of the global irradiation (fig 3). If we assume an own consumption (constant performance) of 320W, as indicated in graphs, this energy would be available from about 8 am until about 5:30 pm which meets a time span of 9 hours and 30 minutes. Projected onto an one-family home with a 5 kWp PV system on its south oriented roof could in this case retrieve a continuous performance of 1600W for 9h and 30min. To be able to actually use this energy, someone would have to remain at home, which is seldom the case since most people are usually at work during that time of the day.

Performance curve of panels in south orientation standardized to one kWp during sunshine on May 26th, 2012

Fig 2: Performance curve of panels in south orientation standardized to one kWp during sunshine on May 26th, 2012

Performance curve of panels in south orientation standardized to one kWp during sunshine on May 26th, 2012

Fig 3: Performance curve of panels in south orientation standardized to one kWp during sunshine on May 26th, 2012

 

In the second case, we chose a considerably cloudy day. As you can see in fig 4, the steady performance we assumed could not consistently be reached. Due to shading from the cloudy skies there were repeatedly performance collapses. Again, we integrated the global irradiation (fig 5) as a validation factor.

The lacking energy referring to a continuous energy demand of 320W must additionally be obtained from the public grid, provided no storage media is being used.

Performance curve of south oriented panels standardized to one kWp during considerably cloudy skies on May 15th, 2012

Fig 4: Performance curve of south oriented panels standardized to one kWp during considerably cloudy skies on May 15th, 2012

Global irradiation on May 15th, 2012

Fig 5: Global irradiation on May 15th, 2012

3. Yield of east- plus west oriented roof

Parallel to our measuring of the south oriented panels, the data readings of the east-west panels were recorded. All three roofs were equipped with the same nominal panel capacity. Again we chose one sunny and one heavily overcast day.

The panels were assembled at an inclination angle of 30° and a deviation of 90° towards east, respectively west. According to the table (see fig 1) this yielded 1% of about 82%. And once again, we used the global irradiation as reference value.

The magenta colored graph in fig 6 displays the performances of the panels facing east. The blue colored graph shows the performance of the panels facing west and the orange graph displays the added overall performance. All standardized to one kWp.

As already done with the south alignments, we mapped an own consumption of 40%, though once referring to the south oriented roof and once to the overall performance of the east-west-oriented roof. On this particular day, neither the east nor the west oriented sides of the roof could quite reach peak performance of 800Wp. We can nevertheless assume that the 560W of our case study match closely the 40% of the peak performance of our east-west-roof.

The upper dashed line in fig 6 shows the app. 40% own consumption of the east-west system. As can be seen, the 40% own consumption of about 560W referring to the overall performance of the east-west roof remained available for about one hour longer than on a solely south oriented roof. In our case, solar electricity was available from 7:50 am until 6:45 pm, which conforms to a time span of 10h and 55min.

The bottom dashed line shows the 40% own consumption of a south oriented roof. This small amount of energy performance is considerably longer available on the east-west oriented roof, hereafter from 6:45 am until 8 pm, which adds up to a time span of 13h and 15min. This amounts to 3h and 45min more than from a solely south oriented roof. This would be a greater possibility for working people to utilize the self produced electrical energy in their own home even without using storage media.

Performance curve of panels in east-west-alignment standardized to one kWp during sunshine

Fig 6: Performance curve of panels in east-west-alignment standardized to one kWp during sunshine

 

As well as for the south oriented roof we opted for a heavily overcast day (see fig 4) for our east-west oriented roof to record the performance curves. As you can see in Fig 7, we again could not continually establish a durable performance. Yet in comparison to the south oriented roof the deficiencies were not as high and not as long. Still, even in this case, energy has to be obtained from the public grid since the demanded continuous power rating isn’t permanently available. Again in this case energy storage could fill in the demand gap by providing energy when panels have performance collapses due to shading caused by clouds.

Performance curve of panels in east-west-orientation standardized to one kWp during considerably cloudy skies

Fig 7: Performance curve of panels in east-west-orientation standardized to one kWp during considerably cloudy skies

 

4. Comparison of south versus east-west

In the following chapters we’ll elaborate the pros and cons of both variations of roof occupancy to enable an overview demonstration.

4.1 Comparison of yields

When comparing the different roof orientations we noticed that when all roofs are equipped with the same panel performance, the east-west system proved a higher yield in an annual average than the south oriented system (see chapter 6). But investment costs (especially in this case) are also about twice as high. It also should be noted, that in spite of double the amount of installed kWp, the peak value performances from an east-west roof (fig 6) are not twice as high than from a south oriented roof (fig 2).

In fig 8 we compared the day’s performances of May 26th, 2012 of both systems and discovered a performance plus from the east-west roof of over 90%. This is most likely caused by minor clouds at peak solar altitude and thereof conditions for the south oriented roof were not quite perfect at the best expected time of day. In a long-term average we assume a yield of approximately 164% from the east-west roof with a double of installed capacity compared to the south oriented roof.

Comparison of yields of south versus east-west on May 26th, 2012

Fig 8: Comparison of yields of south versus east-west on May 26th, 2012
Comparing yields on May 15th, 2012 when heavily overcast, we detected an even higher difference in yields of slightly more than 100% (see fig 9). This is probably explained by the fact that from about 10 am until 1 pm there was hardly any solar irradiance. We once again noticed the peak performance of the east-west system is generally not twice as high as from the south oriented system (see fig 4 and fig 7), still true, especially for May 15th, 2012 the yield of the east-west roof was twice as high as for the south oriented roof.

Comparison of yields south versus east-west on May 15th, 2012

Fig 9: Comparison of yields south versus east-west on May 15th, 2012

4.2 Economical view without storage media

To view the systems not only in terms of energy, we also disclosed a view in terms of economy.

Let’s begin with the south oriented roof and assume a 5kWp system and an own consumption of 40%. The table below adds a few values to the basis of our economical point of view, such as:

Annual average [kWh/kWp] 950
5 kWp system [kWh] 4.750
40% own consumption [kWh] 1.900
Table 1: Base value for our south oriented system

Certainly the electricity rates will rise and not remain steady for the next years
We therefore implemented two calculations for the timeframe of 20 years in which we assumed an annual price increase of 3% in one and an increase of 6% in the other. Acquisition costs of a PV system presently (effective June 2012) range at about 1.500,- € per kWp including project planning, assembly and implementation.

net price gross price (incl. taxes)
Acquisition costs for a 5kWp south oriented system -7.500,00 € -8.925,00 €
40% own consumption (annual expenses +3%) 11.231,82 € 13.365,86 €
Profit 3.731,82 € 4.440,86 €

net price gross price (incl. taxes)
Acquisition costs for a 5kWp south oriented system -7.500,00 € -8.925,00 €
40% own consumption (annual expenses +6%) 15.376,36 € 18.297,89 €
Profit 7.876,38 € 9.372,89 €
Table 2: Balances with virtual energy price increments for south oriented PV system
As shown above, south oriented roofs constitute major cost savings. Managing an own consumption of 40% is difficult though, since the energy required is only available for a comparatively short period of time during the day. In comparison to that we now take a look at an east-west oriented roof. Again we assume 3 and 5 kWp for both roof areas in a system which indicates an overall system size of 6 respectively 10 kWp and an own consumption of 40% of the overall performance. An east-west system yields 82% for each roof side which adds up to 164% of the annual yield of a south oriented roof. This results in following yields:
Average annual yield [kWh / kWp] 807,5
6 kWp system [kWh] 4.845
10 kWp system [kWh] 8.075
40% own consumption for 6 kWp [kWh] 1.938
40% own consumption for 10 kWp [kWh] 3.230
Table 3: System values of an east-west oriented roof

Calculations with +3% and +6% annual price increments are subsequently listed below:
net price gross price (incl. taxes)
Acquisition costs for a 6 kWp south oriented system -9.000,00 € 10.710,00 €
40% own consumption (annual expenses +3%) 11.052,11 € 13.152,01 €
Profit 2.052,11 € 2.442,01 €

net price gross price (incl. taxes)
Acquisition costs for a 6 kWp south oriented system -9.000,00 € -10.710,00 €
40% own consumption (annual expenses +6%) 15.130,36 € 18.005,12 €
Profit 6.130,36 € 7.295,12 €
Table 4: Balances for a 6 kWp system
net price gross price (incl. taxes)
Acquisition costs for a 10 kWp south oriented system -15.000,00 € -17.50000 €
40% own consumption (annual expenses +3%) 18.420,18 € 21.920,01 €
Profit 3.420,18 € 4.070,01 €

net price gross price (incl. taxes)
Acquisition costs for a 10 kWp south oriented system -7.500,00 € -17.850,00 €
40% own consumption (annual expenses +6%) 25.217,26 € 30.008,54 €
Profit 10.217,26 € 12.158,54 €
Table 5: Balances for a 10 kWp east-west oriented system
As displayed above, the profit of a 10 kWp east-west oriented system is even greater in spite of higher acquisition costs. Nevertheless, it still bears difficulties disposing off 40% of energy as own consumption or more, if there is no one at home operating appliances and electrical devices whenever power is available.

4.3 Economical view with storage media

Since all the foregone research took place proposing no storage media is used, we hereafter calculate all values again, this time using one storage possibility, i.e. rechargeable battery.

Like before, we begin with the south oriented roof assuming a 5 kWp system with a battery storage system and thus assume 70% own consumption. We subsequently list some values as basis for our economical views:

Annual average [kWh/kWp] 950
5 kWp system [kWh] 4.750
70% own consumption [kWh] 3.320
Table 6: Base values for our south oriented system

We now set up evaluations based on a time span of 20 years, assuming annual price increments of 3% respectively 6%. Acquisition costs of a PV system ranging at about 3.000,00€ per kWp (effective June 2012) including project planning, assembly and implementation.
net price gross price (incl. taxes)
Acquisition costs for a south oriented 5kWp system -15.000,00 € -17.850,00 €
70% own consumption (annual expenses +3%) 19.622,68 € 23.390,26 €
Profit 4.655,68 € 5.540,26 €

net price gross price (incl. taxes)
Acquisition costs for a south oriented 5kWp system -15.000,00 € -17.850,00 €
70% own consumption (annual expenses +6%) 26.908,66 € 32.2021,31 €
Profit 11.908,66 € 14.171,31 €
Table 7: Balances with virtual energy price increments
As can be seen, there are definite possibilities to obtain significant savings. By means of storing energy, it is much easier to accomplish a possible own consumption of 70%. Now we’ll compare it with an east-west oriented roof. Assuming one PV system of 3 kWp for both sides of the roof and one system of 5 kWp for each side of the roof equals an overall system of 6 kWp respectively 10 kWp and an own consumption of 70% of the total performance. An east-west oriented system of 10 kWp in comparison to the southbound system of 5 kWp yields approximately 82% for each side of the roof, which adds up to 164% of the annual yield of the south oriented roof in a long term average This yields the following returns:
Average annual yield [kWh / kWp] 779
6 kWp system [kWh] 4.674
10 kWp system [kWh] 7.790
70% own consumption for 6 kWp [kWh] 3.271,8
70% own consumption for 10 kWp [kWh] 5.453
Table 8: System values of an east-west system with energy storage
Here again we have the evaluations based on a time span of 20 years, assuming annual price increments of 3% respectively 6%. (see table 9).
net price gross price (incl. taxes)
Acquisition costs for an east-west oriented 6 kWp system -18.000,00 € -21.420,00 €
70% own consumption (annual expenses +3%) 19.341,19 € 23.016,01 €
Profit 1.341,19 € 1.596,01 €

net price gross price (incl. taxes)
Acquisition costs for an east-west oriented 6 kWp system -18.000,00 € 21.420,00 €
70% own consumption (annual costs + 6%) 26.478,12 € 31.508,96 €
Profit 8.487,12 € 10.088,96 €
Table 9: Balance of 6 kWp east-west oriented system
net price gross price (incl. taxes)
Acquisition costs for an east-west oriented 10 kWp system -30.000,00 € -35.700,00 €
70% own consumption (annual expenses +3%) 3.2235,31 € 38360,02 €
Profit 2.235,31 € 2660,02 €

net price gross price (incl. taxes)
Acquisition costs for an east-west oriented 10 kWp system -30.000,00 € -35.700,00 €
70% own consumption (annual expenses + 6%) 44.130,20 € 52.514,94 €
Profit 14.130,20 € 16.814,94 €
Table 10: Balance of a 10 kWp east-west oriented system
As displayed again in table 10, the profit is greater than from a south oriented system is spite of greater acquisition costs. Reaching a higher own consumption with storage means is thus much easier. For the future we can assume a significant decrease in costs for electrochemical storage devices, in particular for lithium-ion accumulators.

5. Optimizing own consumption without storage

A higher autonomy from power supply companies is more possible with an east-west oriented roof than with a south oriented roof. To attain a higher own consumption, it is important to know the individual consumer’s behaviors, because the possible own consumption rate strongly depends on their conduct. Several factors play a roll, for instance, whether or not someone is at home during the day or if all residents are gone, and at what times of the day they wish to use electrical power. To attain the best possible use of the self produced power, there are several opportunities. Subsequently we amplify procedures that manage without storage media.

 

5.1 Self initiative

The most inexpensive way to optimizing own consumption is to operate electrical devices like dishwashers, washers, dryers, etc only at times when the solar generator supplies electrical power. Generally, this is only possible if someone is at home. Due to the absence during the times of highest solar altitude this type of optimizing one’s own consumption is not feasible. Yet there are more ways of optimization which we’ll subsequently illustrate.

5.2 Smart Metering

In the future, another way of optimizing own consumption can be an “intelligent” power meter, the so-called Smart Meter. These digital metering devices will in the future replace the commonly known Ferraris meter. These meters enable the individual consumers to track their own consuming behaviors and thus perhaps alter some behavioral habits to that effect. Furthermore, these devices have a data interface enabling them to communicate with appliances and electrical devices such as washers, dryers, dishwashers and so on. Therefore consumers can operate appliances from afar when there is enough solar power available.

6. Island systems with possible highest autarchy

In order to realize a system operating completely off-grid, there are several things to consider: from the construction of the power supply that is being used up to the storage devices that require sufficient capacity to bridge the time-gap when there is no power source available.

The primary consideration should be given to what type of power source is being used. If it is supposed to be solely photovoltaic the arrangement should ensure sufficient stored power to last the winter months. But this also implies a greatly over dimensioned system. This, on the other hand implies that the excess energy has to be discharged somehow or else valuable energy is wasted which would be lacking during winter.

This leads to the next step of storing energy, since it is impossible to reach 100% autarchy without storage. As mentioned earlier, excess power can be stored and thus be used when the sun is not shining. The question is how long a household is to be kept up during times of lacking power source, but the bottom line is only a matter of prize. Exploitations of further renewable power sources such as wind energy also make sense. Since then another source of energy could be used and one wouldn’t be totally depending on sunshine.

But for now (June 2012) fossil fuels such as diesel, gasoline, domestic or petroleum gas have to be resorted to in order to reach 100% autarchy. These can be used in times when there is no energy available from renewable energy sources and batteries are exhausted. Then a combustion motor could possibly be operated to supply the required power.

But primarily the consuming behaviors of the user have the largest impact on the autonomy of the consumer. If he uses the energy at times when it is naturally available, the storage devices can be configured smaller to bridge the times of lacking energy recovery. Also a combustible motor operated with fossil fuel would not necessarily have to startup quite as often, which in return would have a positive impact on the lifespan of the motor as well as on our environment.

Systems with 100% self sufficiency are by far not as economical as for instance systems with only 70% autarchy (effective June 2012) due to the required combustible motor using diesel or gas.

6.1 Real measuring to autarchy

Since in our institute (TEC Institut) measuring series of panel performances oriented east and west have already been carried out, therefore we are now able to arrange an overview for one entire year (also see bachelor thesis from A. Höfling:”Further Development and Optimization of a Photovoltaic Island System”). For comparison we again used our south oriented roof. Standing in for the displayed power demand, we recorded the real demands of a 4-person household for over 12 months (timeframe: April 1st, 2010 thru March 31st .2011). This enabled us to determine the monthly as well as the annual demand of 4.305,60 kWh and the annual dispensation of consumption (fig 10). The first diagram (fig 10) shows a comparison of a 5 kWp south oriented system with a 6 kWp east-west oriented system. The 6 kWp are equally divided into 3 kWp on the east side and 3 kWp on the west side of the roof. As can be seen within the high-yield months, the east-west oriented system yielded more power than the south oriented system. In winter the south oriented system took a scarce lead. We also discovered that from March thru September we yielded more power than demanded. Regardless of whether south oriented (5 kWp) or east-west oriented with each 3 kWp (sum of 6 kWp) systems, during the cold months from November thru February the demand for power could not be met with solar power. However, in October the PV power could barely meet the demand to self sustain the 4-person household.

Yields of a south oriented roof (5 kWp) and an east (3 kWp) plus west (3 kWp) oriented roof in comparison to a daily demand

Fig 10: Yields of a south oriented roof (5 kWp) and an east (3 kWp) plus west (3 kWp) oriented roof in comparison to a daily demand
We once more calculated the same comparison but this time using the results from the east-west oriented system each side equally equipped with 5 kWp, resulting in a sum of 10 kWp (see fig 18). This shows a self-sufficient power supply from March thru October is met. But just like before, the yields during the cold months of the year do not suffice to ensure complete autarchy. In this case the 4-person household can barely be provided with enough solar power for autarchy in February.

Yields of a south oriented system (5 kWp) and an east- (5 kWp) west (5 kWp) system in comparison to the daily demand

Fig 11: Yields of a south oriented system (5 kWp) and an east- (5 kWp) west (5 kWp) system in comparison to the daily demand
Successive diagrams will show the excess power respectively purchase of electrical power for the individual months for every type of system. We clearly see how much solar energy is being fed-in the public grid respectively has to be purchased from the power company. Just as in previous diagrams it is clear that the additional power demand in winter is significantly less than the excess produced in summer.

As displayed in fig 10 thru fig 14, an achievable self sufficiency from examples with batteries (useable storage 12 kWh) is evident (see table 11).
Orientation of PV- system Size (overall) of PV-system Months of complete autarchy Months of 90% autarchy Monthly purchase (% of annual demand) Reachable autarchy
South 5 kWp 8 – 28 % 72 %
East-west 6 kWp 7 1 32 % 68 %
East-west 10 kWp 8 1 25 % 75 %
Table 11: Overview of different systems
Conclusion: Self sufficiency of 72 %, 68 % and 75% are possible, depending on system size.

As mentioned earlier, it will be quite difficult to manage 100 % self-sufficiency with solar power (effective: June 2012), because during the few months in winter there is insufficient solar irradiation within our latitudes. We therefore believe that complete autarchy from the public grid can only be made possible with a mix of renewable energies. However, these should be put to use only in specific sites, meaning that in regions with little wind it should be avoided to focus on wind energy.

Excess and purchase of power with a south oriented system with 5 kWp

Fig 12: Excess and purchase of power with a south oriented system with 5 kWp

Excess and purchase of power with an east (3 kWp) plus west (3 kWP) system

Fig 13: Excess and purchase of power with an east (3 kWp) plus west (3 kWP) system

Excess and purchase of power with an east (5 kWp) plus west (5 kWp) system

Fig 14: Excess and purchase of power with an east (5 kWp) plus west (5 kWp) system

7. Own consumption with electrochemical batteries

Subsequently we dwell on optimizing own consumption with electrochemical batteries, outline pros and cons of individual systems and how these individual systems have to be designed.

 

7.1. Conventional battery technologies

7.1.1 Lead-acid batteries

Lead-acid batteries are among the oldest electrochemical storage devices. In1854 Wilhelm Josef Sinsteden, a German physician and physicist developed the first lead-acid battery. Due to the long history of the lead-acid battery there already is a lot of experience of stationary use (for example the battery system at Steglitz, the combined heat and power site). As all systems, the lead-acid technology offers some advantages but also some disadvantages which are listed below:

Advantages:

  • Low budget
  • Closed batteries (so-called dry-cell batteries) broaden the spectrum because they are maintenance-free and have a better life-span
  • High economically recyclable components

Disadvantages:

  • Low energy density
  • Not storable when discharged, due to sulfate building up on both electrodes
  • high weight
  • low lifespan, e.g. compared to Li-ion accumulators
  • Necessary maintenance (lead-acid)
  • Separate and ventilated storage room at specific performance categories

Schema of lead-acid battery [2]

Fig 15: Schema of lead-acid battery [2]

 

Lead-acid batteries should preferably be operated fully loaded to reach the possibly highest lifespan. In practice, this is hardly feasible. On solar systems during the summer months we have fragmented cycles with low DODs (depth of discharge), while in cooler and less sunny days we have larger DODs up to full cycles. These irregular loads diminish the lifespan of our batteries.

When designing the individual system performance one should note that half of the nominal capacity is available as useable power in order to increase the lifespan.

 

7.1.2 Redox-Flow

The Redox-Flow cell is a storage media with potential. Such cells know no self-discharge nor do they have a memory effect. Therefore they are able to store energy from renewable sources. Since electrical power is stored in liquid form, the lifespan is significantly higher, because no structural changes take place on the electrodes. Furthermore, large quantities of the electrolytes and thus energy can be stored inside waterproof containers, i.e. barrels or drums.

Advantages:

  • High efficiency (>75%)
  • Long lifespan
  • Minor degradation for each cycle
  • Flexible modular design
  • Quick load suspension (µs-ms)
  • Charge / discharge (0% – 100%)
  • Low maintenance costs
  • Minor discharge

Disadvantages:

  • High energy density (volumetric, gravimetric)
  • High capital costs

Schema of a Redox-Flow battery

Fig 16: Schema of a Redox-Flow battery [2]

 

7.2. More accumulation technologies

7.2.1 Li-Ion Technology

Lithium ion accumulators are characterized by high energy density. They are thermally stable and have no memory-effect. Other than lead-acid batteries which should always be operated fully charged, Li-ion batteries have a different technology. They age slower, even when operated partially discharged. In this case though, it strongly depends on the chemical composition. Therefore the stationary systems should be oversized in order to still operate properly at a low SOC (state of charge).

Advantages:

  • High energy- and power-density
  • High cell voltage (3.6 V – 3.7 V for each cell)
  • High energy efficiency (>90 %)
  • High development potential
  • No maintenance necessary

Disadvantages:

  • Electronic surveillance necessary during operation

Schema of a Li-ion accumulator

Fig 17: Schema of a Li-ion accumulator [3]
7.2.2 Nickel battery

Just as lead-acid batteries, the nickel technology is also somewhat older. However, due to high commodity prices it initially was not often used. It took until the mid-20th century and further advancements in technology for the nickel cell to have a break-thru in the market. Just as with other batteries the performance of this type strongly depends on the chemical composition. Subsequently we address two technologies, one, the nickel-cadmium (NiCd) and the other, the nickel metal hydrate battery (NiMH). Due to prohibitions within the EU, the use of nickel cadmium batteries is very limited and thus this type of technology will most likely disappear from the market completely on long terms.

Advantages:

  • Availability
  • Recyclable
  • Suitability in the HEV (hybrid electric vehicle) operation already established
  • Simple battery management systems (BMS) (no charge balancing system necessary)

Disadvantages:

  • Limited potential for cost reduction
  • Limited potential for technological advancements
  • Performance above 50°C critical
  • Lower specific performance and power in comparison to Li-ion accumulators
  • Cadmium (Cd) is poisonous and must be treated as hazardous waste

8. Optimization of own consumption with other storage media

8.1. Hydrogen

An optimization of own consumption could very well be achieved by producing hydrogen from excess PV power. However, to this day this is not yet practicable for the common user because it still entails some (hazardous) risks. Two carriers of energy, hydrogen and oxygen can be produced from water by electrolysis. This is done by means of electrical current. But now there is a problem of storing. Even if it is collected directly inside a container succeeding electrolyses, it still has to be compressed in order to collect greater amounts in a pressure vessel. Currently, this system is not yet practicable in private homes, but should be possible in about 3 to 5 years.

 

8.2. Methane

Producing methane would be a similar procedure. Here too, water is transformed to hydrogen and oxygen by electrolysis. But now CO2 is added to the hydrogen and thus liberates methane (CH4) This could be stored and used to operate a methane gas driven generator at times when there is no solar power available. If no generator is to be used, methane could also be fed-in the public gas grid. Since only test systems of this type have been operated until now (effective June 2012) it could also take another 3 to 5 years until it could effectively aid renewable energies.

 

8.3. Methanol

Producing methanol takes it one step further. Here methanol (CH4O) is produced from (CH4). Advantages of methanol are quite obvious, because it is much easier to store, transport and use than methane. It also offers advantages compared to premium gasoline with octane ratings of 95 – 98, while methanol reaches an octane number of 133. Methanol in this form is renewable and CO2 neutral.

 

8.4. Compressed air

Compressed air reservoirs are also quite interesting as a medium for storing energy. This technique has existed for awhile. A good example for it is a compressed air storage power plant founded in 1978 in Huntdorf, Germany (see fig 18). The problem with this type of storing is the reservoirs occupying enormous space. Therefore this type of technique makes probably more sense in the area of off-shore wind farms since in northern Germany there are many underground salt domes available as compressed air energy storage space. This method can be used to avert peak power demands with which power plants can be operated at optimum operating point to decrease wear out. The conclusion of the above mentioned processes is, that they should preferably be used at large-scaled industry, rather than in private homes, since they are still economically not of interest to homeowners (effective June 2012).

Compressed air storage power plant

Fig 18: Compressed air storage power plant [5]

 

8.5. Flywheel energy storage

Accumulating energy with a mechanical storage device is also possible. Flywheel energy storage technique is already being used inside large sites, i.e. in the USA. There they are used to improve the grid to intercept frequency fluctuations. One great advantage of such storing method is the ability for multiple million cycles. Moreover, they can absorb and release large amounts of high energy in very little time. However, this type of energy storage bears one tremendous disadvantage, that is their quick self-discharge. Even for systems with a magnetic bearing, installed inside an evacuated room, the stored energy diminishes very fast due to friction. For private home-owners, flywheel energy storage is also not of interest.

 

8.6. Pumped hydro storage

Pumped storage hydro power stations (see fig 19) generally operate almost like conventional storage power stations, except for the upper reservoir isn’t naturally fed. Water from the bottom reservoir gets pumped into the upper reservoir. This takes place at times in which an excess of electric energy is at hand. When energy is demanded from the grid, it can be made available by water in the downpipes actuating the turbines which drive a generator that feeds the produced energy into the grid. These systems have a high efficiency of approximately 75% One disadvantage of these systems is the enormous space requirements and the immense interference in the environment. In Germany, building new pumped storage hydro power stations is quite difficult due to a lack of space. However, in other countries, where space is available, building these stations implies a good alternative to absorb excess power in the grid and releasing it again when demanded.

Schema of a pumped storage hydro power station

Fig 19: Schema of a pumped storage hydro power station [6]

8.7. Water heating

Another variety of using excess energy is a boiler. Here, excessive solar energy is used to heat water inside a boiler with a heating resistor to meet the daily demand of warm water. These systems generally do not suffice to meet the entire demand of hot water they however contribute a great deal to decrease the heating costs.

 

8.8. Refrigerators / Freezers

Another idea of storing a surplus of PV energy could be via appliances which keep cold for a long time, i.e. freezers. They should be configured to the effect of drawing PV power for cooling when it is available, respectively an excess is present. This could also increase the own consumption.

 

8.9. Air conditioning units / air conditioning systems

They are primarily needed when solar irradiation is at its peak.

 

8.10. Ice-Heating

Using ice as a source of energy initially sounds paradox, but however is possible (see fig 20). When water solidifies to ice so-called heat of crystallization is liberated. This energy can be made available for heating a house and for heating service water via a thermal heat pump. The emerged ice can be used for cooling the house in summer which increases the efficiency, because now the heat inside the house is withdrawn to melt the ice. The withdrawn heat is being stored for winter. Furthermore, warmth is being contributed into the system via solar thermal panels to ensure optimal supply in winter. One complete ice-heater consists of the following components: ice storage, air-to-air heat pump and buffer and collector panels for solar heat.
Conclusion: An interesting technique, however very young and thus without long scaled empirical values.

Schema of an ice-heater

Fig 20: Schema of an ice-heater [7]
8.11. CHP (Cogeneration / combined heat and power plant)

A CHP (see fig 21) essentially consists of a motor, a synchronous machine and a heat exchanger. The combustion engine drives the generator for power production. The heat produced is depleted from the coolant and the exhaust gases via a heat exchanger. This certainly is a worthwhile support for the PV system; nevertheless it is not a storage device in the true sense.

Schema of a CHP

Fig 21: Schema of a CHP [8]

9. Conclusion and comments to an east-west oriented roof layout and to the optimization of own-consumption

From figs 10,12 and 13 it is distinctly visible, that an east-plus west roof with each 3 kWp PV performance ( in sum 6 kWp) ranges very close to a south oriented roof with 5kWp in central Germany in reference to the highest possible annual autonomy.

This type of east-west roof layout is a good alternative for home owners that have so far renounced the acquisition of a PV system, due to not having a south oriented roof.

Furthermore, the capital costs of a PV system have decreased drastically that in the mean time it is actually worth (and in the future even more so, due to rising energy bills) storing and using as much as possible self produced PV power.

The east-plus west roof layout ensures an even higher independency from power supply companies.

In households where a parent, children and/or adolescents for example are often home during the day, a very high own consumption can easily be managed.

In addition we presently still receive a feed-in compensation which will range at about 15 Euro cents by December 2012.

Moreover, a PV system will probably significantly exceed a lifespan of 20 years. We generally calculate about 30 to 40 years. The additional 10 to 20 years are not yet taken in consideration.
Recommendation of the TEC institute:

  • South oriented roof with app. 5 kWp or
  • east-plus west oriented roof with app 6 kWp (better: 10 kWp)
  • utilizing batteries
  • Connections should only be carried out by specialized companies
  • Use only current consuming optimized devices
  • Use benefit of remuneration for feeding-in grid, as long as possible
  • Taking in account that a PV system lasts more than 20 years

D List of references

[1] http://www.photovoltaik-web.de/images/stories/PV_Suche/prozentanteil_vom_maximalen_ertrag_in_abhaengigkeit_der_ausrichtung_und_der_dachneigung.jpg
[2] http://de.wikipedia.org/w/index.php?title=Datei:Schema_eines_Bleiakkus_2009-02-09.svg&filetimestamp=20090209222323
[3] http://www.energynet.de/2010/04/21/wind-und-solarstrom-auf-Vorrat
[4] http://www.windkraft-journal.de/2012/04/21update-to-the-rpland-berger-study-on-automotve-li-ion-batteries
[5] http://www.bine.infor/hautnavigation/themen/erneuerbare-energien/photovoltaik/publikation/druckluftspeicher-kraftwerke
[6] http://www.energieroute.de/wasser/speicherkraftwerke.php
[7] http://www.eiheizung.com/wp-content/uploads/20111/01/eisheizung.png
[8] http://www.bhkw.de/de/was_ist_ein_bhkw/bhkw_funktionsschema.html

List of web sources:

[9] http://www.isocal.de
[10] http://batteryuniversity.com/partone-4-german.htm
[11] http://www.eurosolar.de/de/images/storie/pdf/Sauer_Option_Speicher_regenerativ_okt06.pdf
[12] http://www.smart-metering.info/
[13] http://de.wikipedia.org/wiki/Smart_Metering