Using smart storage to enhance rooftop solar performance

September 25th, 2014, Published in Articles: Energize

 

Rooftop , or own generation, solar photovoltaic (PV) systems, are being installed by many commercial and industrial users, including mines, with the aim of reducing energy costs and increasing security of supply. What many users don’t realise is that the load profile results in unused energy which may be utilised to gain further advantages in effective use of the solar power generated. This article examines possible ways to use surplus energy effectively.

Most businesses run a five, or five-and-a-half, day single shift working week with either complete shutdown over weekends or partial shutdown on Saturdays and complete shutdown on Sundays. Even mines have one day a week when the only load is the overhead ventilation and background activities. The solar system however, cannot be switched off and continues to generate energy, which is either wasted, or curtailed to ensure no feedback into the grid. This energy could be stored and effectively used during the working week to reduce total energy demand, lower maximum demand, peak shaving, load shifting and a variety of other demand shaping processes. The advantage of stored energy is that it can be delivered to the load in any profile desired to achieve whatever aim is required if smart storage is used.

Rooftop solar development

Rooftop solar PV has gone through several stages of development as shown in Fig. 1.

Fig. 1: Development of rooftop solar PV systems.

Fig. 1: Development of rooftop solar PV systems.

The first PV systems consisted of panels and inverters and were designed simply to reduce energy consumption. The next generation included energy management features in the inverter to maximise power generation and control the function of the PV system. Once users realised that storage was required, passive storage systems performing a single function were utlilised. The next generation is the use of smart storage which incorporates active power management and allows several different functions to be performed to optimise the use of the system.

Smart storage

Known by a variety of names, such as “powerblocks”, “energy systems”, etc., there are a number of integrated, self contained smart storage systems available on the market. Specifically designed for use with renewal energy sources, these systems incorporate battery storage, inverters, transformers, switchgear and control and monitoring systems. The systems control the flow of energy between the renewable source, the storage, the load, and the grid where grid interchange of energy is allowed. The systems can be programmed to store or release energy in accordance with any desired profile or in response to any conditions. The units are provided in prefabricated containers or shelters and ready to transport to site and range from 50 kVA to 2 MVA in size. The heart of the unit is a bi-directional inverter which controls the flow of energy into and out of the battery, providing controlled charging and discharging. Many units are designed as utility grid connect systems, but an on-site load can be substituted for the grid.

Load profiles

The user’s load profile, the portion of the total load supplied by solar power will determine the extent of the weekend energy and the manner in which it could be used. This usage will also depend on other limiting factors such as:

  • Space limitation: The space available for mounting an array
  • Peak power limitation: The peak power required from the solar system is limited to the peak load

The general form of an industrial or commercial load profile is shown in Fig. 2. There are three important values to consider, and the relative values of each of these will affect the possibility of using stored weekend energy.

Fig. 2: General form of industrial or commercial load profile.

Fig. 2: General form of industrial or commercial load profile.

Peak load

The peak load can have various forms, from the relatively flat load approaching that shown in Fig. 2 with minor variations to complex shapes which may vary depending on the day of the week, weather conditions, etc.

Night-time residual load

This consists of lighting, security and other essential systems which run continuously. In commercial buildings the night-time load should comprise only a small fraction of the daytime peak, whereas in industrial plant the load could be significant, depending on the industry. Examples of continuous loads include refrigeration systems, etc.

Weekend residual load

The weekend residual load could be approximately the same as the night-time load, or even lower. The weekend residual load is often the limiting factor in sizing a rooftop plant, as legislation at present does not allow net-metering or feed-in of surplus power into the grid. It is questionable anyway as to whether the utility would accept “surplus” power over weekends when demand is reduced, or may only do so at a discounted rate. In Fig. 3 the plant has been sized so that the maximum output does not exceed the weekend demand, either by limiting the size, or by curtailing production.

Fig. 3: System size is limited to not exceed minimum load.

Fig. 3: System size is limited to not exceed minimum load.

 In this example the system has been designed to limit the power to less than the weekend residual load, to ensure that no feed into the grid occurs.

Another example is shown in Fig. 4 where the system is designed to provide 75% of the peak load. Assume a weekend residual load of 400 kW and a weekday peak load of 1200 kW. If the array size is not limited and is sized to double the weekend load then the max rises to 800 kW with surplus power available from the weekend.

Fig. 4: Surplus power available from weekend shut down.

Fig. 4: Surplus power available from weekend shut down.

Solar power profile

Peak equivalent hours (PEH)

The concept of peak equivalent hours gives a rough indication of how much energy will be available daily from the solar system. In this method system the solar curve is aggregated and converted to the number of hours at peak or standard solar radiation of 1000 W/m2. This is useful as the panel performance is quoted at a level of 1000 W. To get the PEH take the daily solar radiation and divide by 1000.

Fig 5: Peak equivalent hours.

Fig 5: Peak equivalent hours.

Example:

Radiation at site = 6400 W/m2/day

PEH = 6400/1000 =6,4 h

Array nominal capacity = 50 kW

Approximate energy available = 50*6,4 = 320 kWh/day

Stored energy

In the limiting case the full solar generation capacity over the weekend period can be stored. In practice the decision may be taken to use some of the solar generation to drive the weekend loads and thus only the surplus would be available for storage. How much reduction in grid demand can be achieved will depend on the peak consumption and the size of the solar array. Because the energy is stored it does not need to follow the solar profile but can be applied in a variety of ways to optimise the benefits of the solar PV system.

Performance enhancement

Solar firming

Solar firming involves making the output of the solar plant appear to be constant. Fig. 6 shows how stored energy can be used to reduce the variability of the load profile when solar generates added. Typically, the waveform will produce two peaks at the start and end of the load cycle. Firming adds energy to fill-in the solar production so that the peaks are reduced. Fig 6 shows how energy is added to make the solar curve approach a constant output profile, and hence reduce the peak value of load current or extend the delivery of energy. The extent to which this can be done will depend on the amount of energy stored over the weekend.

Fig .6: Firming is used to modify the shape of the solar output curve.

Fig .6: Firming is used to modify the shape of the solar output curve.

Smoothing out intermittencies

The PV output does not necessarily follow the smooth curve shown in Fig. 6, but will show short interruptions due to passing clouds. As shown in Fig. 7 the larger the solar plant is, the greater will be the effect on the supply. This can effect the peak demand and could have the effect of not reducing the measured maximum demand at all. Rapid changes can cause havoc with the supply of power resulting in rapid voltage variations, frequent transformer load tap changes, surges and other disturbances.

Fig. 7: Smoothing out short interruptions.

Fig. 7: Smoothing out short interruptions.

Energy arbitrage: Time of use tariffs

Energy arbitrage is the storing energy at one time and then discharging at another, effectively shifting energy demand from one time to another. This is commonly used in renewables to describe where surplus energy is generated and stored off peak and then delivered during peak periods, but is also used to describe where energy is stored during times when it is cheap and then discharged during peak metering periods.

It could be beneficial to store solar PV energy over weekends and re-use it during peak tariff times rather than using the solar PV energy during off peak periods. However, where the off peak tariff is low enough it might be worthwhile running the weekend loads off grid power and storing all of the solar energy for use during peak periods.

Eskom Miniflex tariff example (Fig. 8):

Fig. 8a: Eskom miniflex high season weekday tariffs (Eskom[1]).

Fig. 8a: Eskom miniflex high season weekday tariffs (Eskom[1]).

 

Fig 8b Eskom miniflex high season Saturday tariffs ( Eskom [1]).

Fig 8b Eskom miniflex high season Saturday tariffs ( Eskom [1]).

All of Sunday and most of Saturday are considered off-peak times, and electricity used is charged at the off-peak tariff . The peak charges during winter months are five times the off-peak rate, and the standard rate is 40% higher than the off-peak. Peak periods run from 10h00 to 12h00 in the morning and 19h00 to 20h00 at night. The night peak is outside the main usage period of the load, but still falls within the night-time load period, and consumption will still be charged for at this rate. It is also possible to use stored energy to reduce or eliminate the night-time peak hour consumption. Fig. 9 shows how it would be possible to reduce peak hour charges using stored energy.

Fig. 9: Peak tariff period load reduction.

Fig. 9: Peak tariff period load reduction.

Other controls

Stored energy can also be used to modify the ramp rate from the array and in other load profile shaping functions. Two other functions which are possible with smart storage but are not covered by legislation in South Africa are frequency control and reactive power injection. Frequency control will involve either injecting energy into the grid to counter low frequency events, or withdrawing energy from the grid and storing it to counter high frequency or over generation. These functions are done in conjunction with the utility and at a price agreed with the utility.

Design and sizing optimisation

Where the design of the solar system is limited by other parameters, smart storage may be used to optimise the design and enhance benefits from rooftop solar systems. Arrays are often designed in a way to prevent export of energy to the grid during low consumption periods, either by sizing or curtailment. Adding smart storage allows the size of the array to be optimised and surplus energy to stored and re-used as required.

Mounting space limitation

Where the size of the array is limited by mounting space on either rooftop or ground space, stored energy from weekends can be used to effectively increase the output of the array during weekdays, lowering the maximum demand. As shown in Fig. 10 stored energy is added as a block at the start of load, effectively increasing the solar contribution and reducing the maximum demand. Energy addition is shown as a block but could be added in any form or profile.

 

Fig. 10: Stored energy increases the maximum output of the solar PV.

Fig. 10: Stored energy increases the maximum output of the solar PV.

System size limitation

Where the design is limited by the required power output of the system (i.e. supply may not exceed load), smart storage (SS) and weekend generation can be used to reduce the size of the array required. Fig. 11 illustrates this principle, in that the array is limited to the maximum load requirements. Transferring stored energy in block form reduces the array size required while still achieving the same power output, thus resulting in a saving of capital. Where the system is limited to a maximum power output, the stored energy can be used to decrease the size of the array required to deliver the power.

Fig. 11: Reducing the PV array size through the use of smart storage.

Fig. 11: Reducing the PV array size through the use of smart storage.

Cost

One of the common statements made about storage is that it is too expensive. In answer to that it is necessary to ask what the storage is being used for and the value of the benefits obtained. A direct comparison between the cost of storage per kWh and the unit cost of grid power will not yield a meaningful result. Storage is relatively expensive when compared in this manner, but costs are coming down while the cost of grid power is going up, and mass manufacture promises to yield significant price reductions.

Prosperity energy storage project

A similar application which applies all of the principles discussed here is the PNM Prosperity Energy storage project run as part of the US DoE energy storage programme [2]. The project consists of a solar array of 500 kW capacity, a storage unit with a capacity of 1 MWh, and a peak power capability of 250 kW. Eight containerised storage units are used. The main difference here is that cheap energy from the grid is used to charge the storage and not surplus weekend energy as described in this article. The pilot project has shown that all of the performance enhancements are possible. There are reports of several other grid coupled renewable energy storage pilots, but none that I know of which specifically use weekend surplus energy.

References

[1] Eskom :“Schedule of standard prices for Eskom tariffs 1 April to 31 March 2014”, www.eskom.co.za/CustomerCare/TariffsAndCharges/Documents/Schedule_of_Std_Prices_2013_14_excl_Transflex1.pdf

[2] Public service company New Mexico: “Prosperity Energy storage project”, www.pnm.com/solarstorage

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