DC nanogrid for Buildings: Study based on experimental investigation of load performance and Annual energy consumption

Abstract

Considering that most end-use electrical equipments are DC-driven, a thorough examination of the energy delivery system for buildings has become essential. Since alternate energy sources are majorly DC-based, using a DC distribution system instead of a conventional AC distribution system reduces the number of AC/DC conversion stages. This work focuses on the experimental verification of the difference in power consumption between an energy-efficient AC load and its equivalent energy-efficient DC load. Separate case studies are done for two different buildings, comprising mainly of light and fan applications. Since the consumption is majorly for light and fan applications for buildings, 48 V Extra Low Voltage DC (ELVDC) supply is most suited for DC nanogrid. The savings from DC appliances are highlighted, with a focus on actual outputs like illuminance for lights and effective wind speed for fans rather than rpm (the common practice). The energy savings and associated reduction in Green House Gas(GHG) emissions are calculated for buildings of different occupancies. If all the driving forces, viz. renewable energy agenda, loss avoidance in the distribution system and the opportunistic switch to energy-efficient appliances are clubbed together, the benefits are enormous when looked upon with the end destination as a cleaner green planet.

Introduction

Almost every renewable energy sources like solar photovoltaics, wind generators, fuel cells either give output power in DC form or the output power is converted to DC before integrating into the AC grid system. On the other hand, for end-use equipments, especially in residential or commercial buildings, most of the connected loads like LED lamps, BLDC fans, phone and laptop chargers, inverter setup has an internal DC-powered stage. To avoid these multiple conversions from AC-DC or DC-AC, it is better to use a DC nanogrid for connecting these DC loads with the DC source supply. Again, additional storage devices, such as a battery bank, yet another DC source/load, are needed to achieve sustainability due to fluctuations in renewable energy supply patterns and mismatches between supply and demand within the building itself.

Energy-efficient integration of renewable energy supply with end-use equipment is a critical need of the hour. Extra Low Voltage DC (ELVDC) or 48 V DC nanogrid is a potential solution to this challenge[1]. The Bureau of Indian Standards, IEEE SA has recently come up with 48 V DC micro-grid standards and guidelines, which is planned to be presented to the International Electrotechnical Commission (IEC)[2], [3]. Literatures [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16] compare DC and AC distribution performance for building applications. Key points coming out in these studies are better system efficiency considering reduced converter loss or conductor loss, increased battery backup, reduced distributed generator (DG) requirement, optimum DC bus voltage determination. DC architecture provides a lower cost of total ownership over the life, higher reliability, optimised use of solar energy compared to its AC counterpart[5], [17], [18], [19]. Papers [5], [14], [20], [21] focus on the study of DC grid integration in commercial buildings. In [20] line voltage drop and power losses for different DC voltage levels have been compared with the existing AC system. The DC network was found to be economical, majorly due to lesser backup requirement. Literature [5] indicates the increase in photovoltaic performance due to the reduction in conversion state of the power conditioning unit and energy savings due to the reduction in conversion at the load side. Literature [6] considers conversion losses and concludes that DC distribution is advantageous in the presence of DG. On the other hand, in the absence of DG sources, transforming bulk AC power to DC power at the destination is inefficient on its own due to the transformer and associated rectifiers' combined losses. Paper [22] also have similar conclusions. Literature [19] compares AC and DC distribution calculation with the help of conversion efficiency data obtained from various literature and found the reduction in initial investments due to fewer panels and batteries required for serving similar loads. Paper [8] concludes that for low power requirement, higher safety and efficiency for residential DC system, the 48 V system gives optimal performance. Paper [9] compares 24 V and 48 V DC with 230 V AC and prove that total energy consumption is the lowest for 48 V DC systems with optimised cable area. Paper [23] demonstrates that 48 V DC is suitable for lighting and low-power appliances than 24 V and 120 V DC. Paper [12] deals with voltage standardisation of DC supply for residential buildings, simulation studies for efficiency analysis and concludes that the DC system is more efficient. Paper [24] states that 48 V systems are more suited to customers having low essential loads (<500 W) where a battery acts as backup and recommends a 380 V system to consumers with higher loads (>1kW). Article [25] elaborates the Demand Side Management (DSM) System for DC grids inside the building, which improves the system efficiency by managing energy inside the DC grid. Literature [11] discusses LVDC architecture for residential application and a comparison study was done on energy consumption between conventional high power consuming AC loads and DC powered energy efficient loads. In[14], [15] a simulation-based energy comparison and economic analysis is performed for Zero Net Energy Building(ZNE) but it concludes that DC distribution is best suited only for the buildings with large solar capacity and high battery capacity. In [13] detailed computational comparison is done considering similar DC and AC loads categorising the loads into AC load, DC load & independent load types and various case studies are performed. The differences were minimal but highly dependent on the efficiency of the power transfer system. The literature [26], studies on appliances, focuses on DC and AC's coexistence as a source. The AC sourced appliances like CFL and LED, office equipment like computer, TV and motor drive were replaced by a 180 V DC source keeping the rest untouched, and the performance was compared with a 220 V rated AC sourced set up. The paper concludes that power quality in the steady-state operation is improved due to DC system characteristics. Paper [27] compares AC and DC LED lights' efficiencies, driver efficiencies, and thermal stability. A life test of DC LED light was also performed. The conclusion is that the DC lights are more efficient and reliable than AC lights. Lights of different ratings are compared and the efficiency of a typical system was studied. The reason for reducing cost, higher efficiency, and DC LED lights' advantages are explained.

The present study compares the efficiency of a set up with 48 V DC LED lights, and fans with AC fed LED lights, and energy-efficient AC fed BLDC fans of similar specifications experimentally. Two types of loads were considered for the case study. The first two buildings were already using energy-efficient appliances, one with group B occupancy type, the second with group E occupancy type. Annual saving was arrived at by replacing existing energy-efficient AC devices with energy-efficient DC devices and matching the delivered performance characteristics.