^{1}

^{1}

^{1}

^{2}

^{1}

^{2}

The Low Voltage Direct Current (LVDC) system concept has been growing in the recent times due to its characteristics and advantages like renewable energy source compatibility, more straightforward integration with storage utilities through power electronic converters, and distributed loads. This paper presents the energy efficiency performances of a proposed LVDC supply concept and other classical PV chains architectures. A PV source was considered in the studied nanogrids. The notion of relative saved energy (RSE) was introduced to compare the studied PV systems energy performances. The obtained results revealed that the use of the proposed LVDC chain supply concept increases the nanogrid efficiency. The installed PV power source in the building should be well sized regarding the consumed power in order to register a high system RSE. The efficiency of the new LVDC architecture is 10% higher than the conventional LVDC one.

The photovoltaic electricity was primarily established for standalone applications deprived of any connection to a power grid. Such was the case of satellites or isolated habitations. Currently, PVs are found in many power applications like personal calculators, watches, and other objects of daily use; they can supply many individual DC loads without difficulty. Due to the evolution of photovoltaic systems connected to the grid, the PV has been considerably exploited as a solution to produce electricity.

The main objective of this paper is to investigate the energy efficiency performance of a proposed Low Voltage Direct Current (LVDC) PV system regarding a classical LVDC architecture and classical PV systems using AC loads. All the studied PV chains are on-grid ones and are supposed to be supplying offices.

In general, electric energy consumption in office applications and housing is achieved by using the alternative current plugs even for Grid Tie PV panel systems. In this case, the use of ACs can increase system loss especially when DC current is used at the load levels.

LVDC systems have been gaining more interest during the past few years in both academia and industry. LVDC systems offer many advantages covering higher energy efficiency and easier integration of modern energy resources in comparison to conventional AC systems. Multiple factors affecting the reliability performance and power quality of the electricity supply permit the use of DC systems instead of AC systems. Direct use of DC power would reduce many power conversion losses by exploiting self-consumption of the energy produced on site and decreasing imports of electricity from the grid. DC loads used in households and office buildings also operate on DC; heating/cooling systems and larger equipment used in industry such as variable frequency drives have also adopted DC motors. Direct current power systems are essentially more efficient than their AC counterparts, since in DC systems do not suffer from skin effect or reactive power [

A literature research has exposed the study of the first system analysis that explored the use of very low voltage (<120 V) in small-size systems, particularly residential dwellings [

Subsequently, Lasseter proposed the concept of the DC Microgrid as a low voltage distribution network. This concept was projected as the future low voltage distribution systems which were facing revolutionary variations at the time due to emanation of distributed generation and market liberalization. The basic idea behind this concept is to combine micro sources and loads into one entity which could be interpreted as a single dispatchable load that could respond in short time to meet the transmission system needs [

For many years, The LVDC system has been developed for specific applications like aerospace, automotive, and marine [

Main DC voltage levels and applications.

Voltage level | Applications |
---|---|

1500 V | PV systems, traction systems, marine, aircraft system, LVDC distribution system. |

750 V, 400 V | Industry and transportation (metro, Tram power systems) |

380 V | Standard in the data center industry, buildings, commercial buildings centers [ |

48 V | Telecom, small PV systems [ |

24 V | Lighting systems (LED) standards for DC distribution, according to industry experts interviewed. DC-house DC–DC converter: 380 _{DC}/24 _{DC} [ |

12 V | Lighting, automotive industry [ |

5 V | Microprocessors and electronics [ |

Adopting direct current in data centers improves efficiency, decreases capital cost, increases reliability, and boosts power quality [

The most significant challenge that DC distribution systems face today is the lack of standardization inducing varied architectures and operations of DC distribution systems [

A recent analysis conducted by Vossos et al. [

The work presented in [

The daily energy savings are intensely correlated with the consumption of residential load profile. Hence, the peak PV production can only be attained during the midday, while demand for electricity peaks is in the afternoon and evening. Energy storage systems can store extra solar power for later use and avoid DC/AC and AC/DC conversion losses of transmitting the extra electricity on the grid. Studies conducted in [

The authors of [

Hence, the losses are multiplicative and not additive. An uninterruptable power supply (UPS) uses the battery to keep supported loads up. For storage in a battery, power is converted from AC power to DC. For the server plug, DC/AC conversion is crucial. The processors use the DC power achieved by the server so multistage conversion structures (AC/DC/AC/DC) are used in this case.

The DC power is directly produced from residential solar panels and inverter is commonly added to supply AC loads.

Despite that, the multistage conversion is basic to extract power from the solar panel into the server; losses resulting from these conversions are expected to be between 10% and 25%.

Through review of the available literature [

Ammous and Morel [

The majority of works on DC distribution grids assume that converters are installed at each household, which connect the local DC or AC nanogrids [

The paper is organized as follows: In the first part, we focused our study on the state of the art related to the use of the LVDC supply concept and the proposition of an on-grid LVDC PV chain. The disadvantages of the use of classical on-grid PV systems and of using AC plugs to supply electric DC loads are shown. The used average model of power converters is then presented. This model allows the evaluation of the different converters efficiencies in the studied PV chains. The last part of the paper treats the energy efficiency performances of the proposed LVDC system compared to others classical ones. For this purpose, offices loads are considered and Jeddah location (in KSA) was chosen in our study.

Electronic appliances, such as computers, gaming consoles, printers, economic or LED lights, televisions, and so on, need DC supplies. Additional AC to DC converters are needed in such equipment.

Figure

Classical configuration (syst1) of the on-grid LVAC PV system and DC load powered from AC sockets.

The DC/AC converter, Converter #2, transfers the PV generated power to the grid and ensures the regulation of the DC voltage value (400 V) of the inverter input. This DC value is mainly used value for single phase PV systems allowing easily obtaining the standard 230 V AC. The injected current to the grid has a quasi-sinusoidal waveform.

The main used and conventional LVDC photovoltaic architecture is shown in Figure _{3}) adapts LVDC loads supplies to this DC bus.

Possible LVDC classical architecture (syst2).

The efficiency of each PV conversion chain depends on different considerations like the type of used power semiconductor devices and the magnitude of the transferred power.

Circuit oriented simulations on Saber tool (from Synopsys) allowed generating the evolutions of different converters efficiencies as a function of transferred power.

Figure

On-grid PV chain (syst1) implemented in “Saber” simulation tool.

The evolution of the injected current into the grid, by the PV system, is shown in Figure

Evolution of the injected current (magenta) into the grid and its reference (blue).

In what follows, we present the proposed new PV architecture for DC loads supplies and the developed average model of power converters. The proposed LVDC PV chain uses the DC bus available directly after the PV panels. This bus is not regulated but its value varies in a given range depending on PV panels associations (parallel/series), open circuit voltage across each panel, and the operating point in the

The regulation of the DC voltages supplying loads is regulated by DC/DC converters (in general Buck ones). The proposed architecture is the one shown in Figure

Proposed LVDC architecture (syst3).

Modeling is required to analyze the dynamic behavior of a power converter in several applications. Both accuracy and simulation rapidity are essential particularly for long time simulations and for complicated circuits. The averaging method is the widely used technique for complex power electronics systems. Based on the classical averaged model, the converter is considered to be a linear system using ideal switches. Contrarily, the nonlinear averaged model is established on semiconductor device models including static and dynamic characteristics of the switches as it was used in our case. Figure _{1} in series with a controlled current source _{1} given by_{as}(_{e2}(_{s} (switching period of the controlled switches).

(a) The PWM switch. (b) The corresponding averaged model.

Figure _{as} (_{e1} (_{bs} (_{e2} (_{s}. The driving signals _{1} and _{2}, respectively.

Adopted switching characteristics for the controlled switch and the diode components in the PWM-switch cell.

The power losses of semiconductors (_{switch} and _{diode}) are estimated using analytical representation of the switching characteristics which include both conduction and switching losses and considering the various conduction and switching times.

Different static and dynamic power devices parameters can be deduced from their data sheets or by experiments. The developed average model allows computing all the dissipated power in the semiconductor devices and then deducing the different converters efficiencies. Time domain simulations will be more rapid for the whole PV chain system.

The efficiencies of the different converters used in the different chains were evaluated by mean of refined simulations. The evolutions of these efficiencies as a function of the transferred power

Different converters efficiency.

We defined the saved energy, _{i} (^{2} irradiance conditions.

Jeddah-KSA location (21°32′ 34″ N, 39° 10′ 22″ E) was chosen to perform energy efficiency performances of the different studied PV chains. Jeddah features an arid climate under Koppen’s climate classification, with a tropical temperature range. Unlike other Saudi Arabian cities, Jeddah retains its warm temperature in winter, which can range from 15°C (59°F) at dawn to 28°C (82°F) in the afternoon. Summer temperatures are extremely hot, often breaking the 48°C (118°F) mark in the afternoon and dropping to 35°C (95°F) in the evening. The lowest temperature ever recorded in Jeddah was 9.8°C (49.6°F) on February 10, 1993. The highest temperature ever recorded in Jeddah was 52.0°C (125.6°F) on June 22, 2010.

The following graphs in Figure

Solar irradiance of typical days for each month in seasons. (a) Winter, (b) spring, (c) summer, (d) autumn.

The following graphs in Figure

Temperature of typical days for each month and season. (a) Winter, (b) spring, (c) summer, (d) autumn.

The generated power by the panels during a typical day of January in Jeddah is shown in Figure _{PV} is equal to 3.006 MWh. This generated energy was calculated based on irradiance, ambient temperature, and wind speed in Jeddah during each typical day/month in the year.

Generated power, by the used panels, during a typical day of January in Jeddah.

In what follows, we will describe the assessment of the efficiency of the proposed LVDC PV chain solution (syst3) compared to the LVDC classic one (syst1) and the classical on-grid PV chain (syst2) using AC loads.

First, a load profile of an office was chosen in order to make comparison of the saved energy by the three studied PV chains. The profile of the load is shown in Figure _{Load} = 1.068 MWh.

The assumed daily office load consumption (by 1 office).

The considered office load is composed due to desktop computer, laptop, laser printer, two led lamps, small TV, and a fan. Since the DC/DC converter (_{3}) is common for the three studied converters and located just before the load, its effect was not taken into account in the study.

The saved energy by the three models during each month of the year in Jeddah city is shown in Figure

Annual saved energy by the three PV chains models (classical LVAC on-grid (syst1), conventional LVDC (syst2), proposed LVDC (syst3)) for each month of the year in Jeddah city (for (a):one office and (b): two offices’ load).

From Figure _{3} = 1.467 MWh/year when only one office load is considered) compared to the other chains (_{2} = 1.357 MWh/year and _{1} = 1.319 MWh/year when only one office load is considered too). This improvement of the saved energy is due to the localization of the load connection in the PV chain.

In fact, in the proposed system 3, the load is close to the PV panels and a short traveled path of the energy will be registered. We note that when the saved energy _{i} is negative, this means that the consumed energy is higher than PV generated one and so, this energy is transferred from the grid to the load.

We varied the consumed energy by the load (more offices) and we registered the saved energy by each PV system. It was remarked a very interesting propriety of the PV LVDC systems related to the increase of their efficiency compared to classical PV chain (system 1).

In fact, we define the relative saved energy (RSE_{j} %) of the PV LVDC chain (syst _{2} or _{3}) regarding the classical PV system (_{1}) by the value of the annual generated PV energy _{PV}.

The defined PV system RSE_{j} with respect to the classical LVAC PV chain (system 1) is given by the following equation:

If the defined efficiency is negative this means that classical on-grid PV chain (system 1) using AC sockets for load supplies is better (in terms of energy saving) than the PV LVDC supplies (systems 2 and 3) concept.

Figure

Relative saved energy for the two systems, 2 and 3, in each month (number of offices = 1).

The waveforms giving the evolution of the RSE_{j} LVDC chains, as function of the rate of load consumed energy by the PV generated energy

Yearly relative saved energy of classical LVDC system 2 and proposed LVDC system 3.

Two main observations can be highlighted when interpreting Figure

First, it is clear that the RSE of the proposed LVDC chain (syst3) is higher than the classical LVDC one (syst2) till a high load consumed energy (_{Load} equal to 2.5 times the PV generated energy _{PV} in our case).

Second, optimums of these waveforms are registered when the consumed energy is around the generated energy by the PV panels. The maximum yearly RSE of the new LVDC architecture (syst3) is about 12% while the one registered by the conventional LVDC chain (syst2) is about 2.3%.

From Figure

We can remark also that, for a high load consumption magnitude, the LVDC chain (syst2) becomes more interesting than the proposed LVDC chain (syst3).

In addition, the use of system 3 allows the increase of the load energy consumption range (more than two times the PV generated energy) where the LVDC supply concept efficiency is higher than other systems.

In the proposed LVDC architecture, the power generated by PVs takes a shorter path heading towards the load directly without going through the power controller (

All the obtained results show that the use of the LVDC chain supply concept is very interesting and the use of DC loads instead of AC loads, when a PV power is generated locally, increases the PV system efficiency especially in the case of the proposed supply concept.

The installed PV power in the building should be well sized regarding the consumed power in order to register a high system RSE. In this case, an LVDC system RSE can be higher than 10% compared to the classical LVAC supply in PV systems.

In this paper we proposed a new architecture of a Low Voltage Direct Current (LVDC) supply concept. The proposed on-grid PV chain system, involving DC loads, can replace the classical on-grid LVAC systems using AC plugs to supply electric AC loads. To evaluate the efficiency of some different PV chains, nonideal averaged models of the different converters have been used. The used models have the propriety to be accurate and suitable for complex systems including many converters. The energy efficiency of the different PV chains was estimated by mean of simulations. The evaluation of the efficiency of the proposed new LVDC architecture compared to the conventional LVDC one was performed in the case of building offices in Jeddah city. The superiority of the proposed LVDC PV chain was shown; it can reach 10% higher than the conventional system and depends on the consumed load energy to PV generated energy ratio. In the future, the performance study of this proposed LVDC system can be done for residential loads and efficiency rate can be discussed and compared to the case of offices loads.

There are no conflicts of interest among the authors.

The authors extend their appreciation to the Deputyship for Research and Innovation, Ministry of Education in Saudi Arabia for funding this research work through the project number 1212.

_{dc}brings reliability and efficiency to sustainable data centers