The Importance of PV Immersion Control

Businesses and consumers alike are quickly beginning to realise that the true value of solar PV lies in the long-term energy cost savings that systems can deliver, rather than just the benefits from the government incentives. This will become increasingly more evident as, over time, financial incentives will become smaller and energy costs will rise.

Together with the introduction of state-of-the art energy management systems and storage options now more widely available, installers can offer an enhanced solar PV installation that will generate tangible energy savings and future-proofing energy supply – in addition to the financial rewards from micro-generation.

Cost effective energy storage has been considered the missing link to ensure the viability of solar PV for the long term. Development of this market has been hindered by high cost, intrusive installation, and limits in the overall lifespan of existing solutions.

As the Department of Energy and Climate Change deems mass export metering to be too complex and costly, owners of solar PV can benefit from an additional export tariff deemed at 50 percent of electricity generated – in combination with their feed-in tariff (FiT). However, market feedback suggests, attention is gradually shifting away from the financial incentives in a move towards energy efficient measures which will effectively reduce reliance on "The Big Six" energy suppliers. Indeed, research firm Nanotech suggests that the solar storage market will be driven by a decline in costs of PV modules and a reduction in Government support for solar power.

Government interventionist policies and measures to stimulate renewable uptake have undoubtedly played a pivotal role in the development of the renewable industry in the UK. By taking heed of our European counterparts, more precise measures can be taken to successfully advocate and promote the uptake of effective renewable energy storage. Germany has recently announced its strategy to subsidise consumers’ purchases of battery systems with which to store solar energy. Considered by many to be a pioneering development, other industry specialists believe this move to be short-term in nature, forecasting budgeted funding to run dry before any large scale impact can be effected.

A recent study by IMS Research reports 2.5 GW of solar PV storage is expected to be installed in the UK by 2015. Its survey of over 400 manufacturers, suppliers and integrators suggests a clear appetite for energy storage primarily amongst residential PV customers who, having made the investment into renewable energy, are striving to maximise their own consumption of the energy they are generating.

Hot Water Storage

Whilst research and innovation in this growing sector continues to progress, the market has to date leaned towards alternative, lower cost solutions to self-generated energy. Hot water storage has been viewed as the next best alternative to battery based systems – storing surplus energy as hot water for use throughout the day, placing less strain on the more common heating methods such as gas; considered as a back-up to our normal DHW procurement.

Cost effective energy storage has been recognised as the key to ensuring the viability of solar PV for the long-term. Whilst this is now seen as the way forward, the ideal installation must also address the technical aspects of power modulation to typical appliances such as an immersion heater.

The UK market has seen a number of devices introduced that boast the intelligence of diverting surplus energy to heat storage. However, research suggests significant differences amongst competitor products – although claiming the same fundamental functionality – suggesting a possible lack of due diligence amongst industry players.

Technology Overview

The underlying concern surrounding such devices is based around the technology adopted, i.e. the modulation employed to alter the AC sine wave, such that the energy diverted to the resistive load can be "proportionally controlled" so as to use only surplus energy available.

The three mainstream methods of modulation have been categorized as Burst Fire, Phase Angle and Pulse-Width Modulation (PWM). In-depth analysis of these technologies has revealed the requirement to address a number of critical issues.

Burst Fire

Burst Fire control, also known as ‘on-off control’ or ‘integral-cycle’, allows a number of complete cycles (or half-cycles) through to the load – by blanking / missing out cycles periodically in sequence. In basic terms, your power switch is turned on (active) at zero mains voltage.

Taking Diagram 1 as an example; by regulating the wave patterns, loads can be modulated by sending waves with single pauses in between. Or, more frequently, by sending two waves with pauses in between them as in Diagram 2.

Diagram 1

Diagram 2

 

Although the simplest form of control, negating the need for in-depth algorithmic programming, difficulties lie in containing the generation of harmonics thereby causing abrupt ‘flickering’ in electrical circuitry.

Phase Angle Control

Phase Angle control, also known as ‘trailing-edge dimmers’, turn off the electrical load at the end, or trailing edge, of the AC sine wave and is designed to modulate power to the resistive load by varying the duty cycle (the ratio of on time to on+off time) of the voltage base to the resistive load – see Diagram 3.

Although recognised to be an inexpensive method of control, due to the dimming characteristics; this method is prone to create a great deal of electrical noise during operations.

Research has found this method of control is normally used in smaller capacity loads – approximately 700W or less. Electrical loads greater than the prescribed average will have difficulties in containing electrical emissions without adequate filters, due to the generation of harmonic waves greater than the intended power size.

Diagram 3

 Pulse-Width Modulation (PWM)

PWM control is designed to splice the AC sine wave in order to modulate the flow of power to an (resistive) electrical load. The key difference in comparison to other technologies is the conformity to the ‘pulse duration’ (53) i.e. return to origin. The slices (52) represent part of the real wave to form a complete wave-length.

Diagram 4

By smoothing over the wave form, this method of control inherently reduces the level of harmonics generated through modulation – inducing little distortion to the overall sine wave. PWM is designed to reduce harmonics in apt wave-forms and enables this by producing a ‘quasi-sine wave’.

Following rigorous testing, 4-noks have found PWM to be the most suitable in altering the energy diverted to the load, whilst causing the least strain on the AC sine wave. With the inclusion of specific filters, harmonic emissions can be contained to acceptable levels, enabling the solution to meet the stringent CE and EMC compliancy for the domestic class.

Legal Requirements

Electromagnetic phenomena and their effects on electrical equipment and circuitry have compelled authorities to implement measures to ensure proper operation of equipment under specific norms (depending on intended use). Of course compliancy criteria and schemes used will differ from country to country but the overall importance holds true.

Within the European Union we are more familiar with CE marking, where rules are written within EU Directives, stipulated by national law – as a consequence, non-compliant equipment can lead to a fine or even imprisonment.

Given the rapid growth in demand within the residential sector for these solutions, devices intended for this market must correspond and adhere to the following directives and norms:

EMC (Electro Magnetic Compliance)

EN 55014-1: Electromagnetic compatibility - Requirements for household appliances, electric tools and similar apparatus - Part 1: Emission;

EN 55014-2: Electromagnetic compatibility - Requirements for household appliances, electric tools and similar apparatus - Part 2: Immunity - Product family standard;

EN 61000-3-2: Electromagnetic compatibility (EMC) - Part 3-2: Limits - Limits for harmonic current emissions (equipment input current up to 16 A per phase)

EN 61000-3-3: Electromagnetic compatibility (EMC) - Part 3-3: Limits - Limitation of voltage changes, voltage fluctuations and flicker in public low-voltage supply systems (equipment with rated current up to 16 A per phase)

LVD (Low Voltage Directive)

EN 60335-1: Household and similar electrical appliances - Safety - Part 1: General requirements;

EN 60335-2-21: Household and similar electrical appliances – Safety – Part 2-21: Particular requirements for storage water heaters.

Lead image: Large solar panel installation on roof via Shutterstock

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Zuber heads up the renewable energy division of Greenologic and has been primarily responsible for the launch of a range of monitoring and control solutions specifically for the Solar PV sector. Building on the wealth of experience gained from 4-noks in Italy, Greenologic now channel their growing product portfolio in the UK and overseas, helping renewable installers and energy management companies add value to their projects. By ident...

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