10 myths and truths about photovoltaic minigeneration projects

Do you know everything about photovoltaic minigeneration projects? At the invitation of Canal Solar, experts responded to 10 statements on the topic, and told us: is it true or myth? Understand why.

1) A ground resistance of less than 10 ohms assures me that the ground design is good

Paulo Edmundo Freire, master in electrical engineering, doctor in geology, rapporteur of the draft Brazilian standard for grounding solar farms and professor at Canal Solar, declared that the statement is a myth.

“It is worth noting that low grounding resistances do not guarantee a safe project, and that high grounding resistances do not necessarily mean an unsafe project. As established by standard NBR-15751/2013 – item 10.1 General Recommendations – the grounding design must guarantee sufficient phase-to-ground short-circuit current levels to allow the backup protection to operate, as well as withstandable step and touch potentials , which can be obtained by a grounding mesh geometry compatible with the local soil resistivity, with the portion of the fault current dissipated by the mesh and with the protection actuation times”.

2) The costs of reinforcements and adjustments to the network are always the responsibility of the energy consumer (accessor)

Dirceu Ferreira, master in electrical engineering, with more than 30 years of experience at the energy distributor CPFL, assessed this statement as a myth, and explained further about the costs of reinforcements and adjustments to the network.

“The consumer's responsibility costs, known as PFC (Consumer Financial Participation) depend on the demand to be contracted, the proportional cost of adjustment works in the system (CP) and the ERD (Distributor's Responsibility Charge). In other words, PFC = CP – ERD. If the ERD is greater than or equal to the Proportionalized Cost (PC), the consumer will have no cost for the adjustments, everything being the responsibility of the distributor”.

3) It is not allowed to sell the energy generated by my solar park

Bernardo Marangon, master in electrical engineering and director of exact energy, confirmed that the statement is true and also explained why.

“In regulations, the sale of energy in the regulated contracting environment is prohibited, that is, where Distributed Generation operates there is no energy transaction, the method used is Net Metering. When the consumer unit injects energy into the grid, this generates a credit in kWh with the energy distributor. When the consumer consumes energy from the grid, this credit is compensated. If the consumer generates more than he consumes, he cannot sell it, this energy will be computed as a credit to be used in the next month”, stated Marangon. “Within the scope of the ACL (Free Contracting Environment) this statement would be false, as energy purchase and sale transactions can be carried out in this environment”, added the engineer.

4) Maintenance costs make the use of trackers unfeasible

Vitor Tavenari, aeronautical engineer and president of Politec, confirmed that the statement is a myth and also explained how these costs work. “The percentage of possible maintenance is very low compared to the possible gains of up to 28% of energy generation, which reduce the cost of the energy generated. The additional maintenance cost is 0.3% of Capex per year. Just comparing the cost of demand, which reduces 20% by R$/MWh with tracker, represents a gain 10 times greater than the increase in cost due to possible maintenance of the tracker”.

5 ) It is possible to bury cables directly in the ground

Mateus Vinturini, electrical engineer and technical consultant at Canal Solar, certified the statement as true and also listed the precautions needed to bury the cables directly in the ground.

“To bury, the cable must be of the reinforced type or its installation must provide some type of mechanical protection above the cable, such as a light layer of concrete or other material that makes it protected against earth movements. Furthermore, it is recommended to contact the cable manufacturer to find out if they confirm that the equipment can be directly buried or if there is another cable better prepared for this in their portfolio”.

6) The lower the cost per Wp, the better the viability of the project

Felipe Santos, materials engineer and sales manager at photovoltaic module manufacturer JA Solar, explained that this statement is generally classified as a myth, but this can vary depending on the system.

“The lower the Wp of the module, or of the equipment in general, means that it is equipment using more obsolete technologies and with lower power. Thus, you will need many more modules for the same system size, so you will save on the module, but you will spend a lot on other components called BOS (Balance of System). Therefore, it will lead to a higher total cost of the system, that is, the plywood on one side is spent on the other. The lower Wp will not always lead to a lower system cost. The ideal is to always calculate the LCOE, which is the cost of the investment, divided by the amount of energy generated in the period. Higher technology and higher power modules will provide a lower BOS and LCOE. However, if it is a residential system it may be a cheaper module, this difference is less noticeable in a rooftop system”.

7) Oversizing my inverter reduces the useful life of this equipment

Ricardo Alonso, electrical engineer and engineering director at Sungrow, explains why the phrase is nothing more than a myth. “This is a mistake or lack of knowledge. Inverters do not suffer from overloading. The important thing is to ensure that the maximum current and maximum DC voltage supported by the inverter are not exceeded.

With current photovoltaic modules, it is possible to reach close to the maximum voltage and close to the maximum current of the inverter, with an overload of 80% to 90%. For example, a 250 kWac inverter could have around 460 kWp connected to its input and this would not cause any problems, because the inverter is intelligent, it will never work at a power above its nominal power (in this case 250 kW).

When the photovoltaic array has a generation capacity greater than the power of the inverter, it switches the operating point on the IV curve of this array to a point of lower power. This is done by the MPPT module, which will act outside the maximum power point of the photovoltaic modules, causing what we call clipping, a power limitation. This way there is no heat dissipation, which is another myth, the inverter simply stops operating at the maximum power point of the modules, and there will be no loss of warranty or reduction in useful life”, explains Alonso.

“It is important to highlight that despite supporting this high level of overload, in most cases it is not financially viable to work with high overloads. To identify the optimal overload, it is recommended to carry out a LCOE analysis. The more you increase the power of photovoltaic panels, the more power generation increases, the capacity factor of the plant increases. However, the cost of investment also increases. You do this until your LCOE is as low as possible. It makes sense to increase the overload until the energy cost is as low as possible. Here in Brazil, in the Northeast region and north of Minas Gerais, for example, which have a high solar incidence, the optimal loading factor is between 36% and 42%. In the South, something around 45% and 50%, adds Alonso.

8) EPC and EPCm are the same thing

Kleber Alota, mechanical engineer and technical director at AK Energia Solar, stated that the two terms are not the same thing and also explained the difference between them. “EPC stands for Engineering, Procurement and Construction. EPC-M stands for Engineering, Procurement, Construction and Management”, explains Alota.

“An EPC supply or contract occurs when the end customer fully delegates the performance of the services and work necessary for the construction of the solar park to a third party company. Generally, this is a very large company, which has in its scope most of the supply chains and what needs to be delivered for the construction of the solar park. If the EPCista company does not have the skills for all the necessary areas and sectors, it subcontracts and outsources services – which is its responsibility to the end client – to meet the entire scope contracted by the client. This type of contracting tends to be the most expensive, financially speaking, as the EPCist assumes all responsibility for carrying out and building the contracted scope”, he adds.

“EPC-M is when the client hires an EPCist company to supply a large part of the necessary scope, but the client itself assumes part of the responsibility for the scope of the project. This will cause the value of hiring EPCistas to decrease, as part of the scope will be covered by the client or another supplier under their management. This type of supply usually has a lower final value for the customer, in addition to having much more effective management, as the manager appears, who is another company and has no connection with the EPCistas company. The manager or management company will ensure that the total scope, both of services and equipment necessary for the construction of the solar park, occurs within the schedule, with the expected quality and budget, meeting all the needs of the end customer”, concludes Alota.

9) The harmonic content generated by my inverters influences the design of the photovoltaic park

Ricardo Alonso also uncovered this myth. According to him, the inverters are severely tested and must always operate within established limits, which are always within the requirements of a solar park. “For this to be true, the inverter must have a problem or the number of inverters in parallel generate resonance. We can calculate it, and such simulation of harmonic content at the connection point should be mandatory.”

10) Driving piles into the ground improves the performance of the photovoltaic park's grounding system.

True. The expert Paulo Edmundo Freire stated this. “The stakes driven into the ground are part of the support structures for the photovoltaic arrays and, together with the grounding mesh, are part of the photovoltaic plant’s grounding system. Even if inside a concrete coating, they act as grounding rods, providing paths to the ground for atmospheric discharges and contributing to the control of step and touch voltages in the UFV area”.