There has been an increase in global concern about energy consumption. Various energy sources have been used over the last few decades. Pollution as well as increased repercussions such as global warming and climatic changes have put a strain on fossil fuels. The demand for energy has been increasing, yet the supply of energy has gotten more expensive due to the depletion of nonrenewable resources. Alternative energies such as renewable energy sources (RES) such as solar energy and wind energy have been proposed. The building sector has been affected by the increased energy demand, The NZEB (Net Zero Energy Building) has been introduced as the new architectural designs for the building. The NZEB is a building design whereby the amount of energy consumed annually equals the amount of onsite produced energy. The energy management in the NZEB involved the use of the grid connections, the REC (renewable energy certificates), building environment niche, and N+NZEB technology. Some countries have embraced the NZEB technology as a building and construction technology. For instance, the European states have formed the energy commissions that regulate the compliance of NZEB requirements by member states. The NZEB technology is a relevant tool for reducing the energy consumption as well as environmental impact of the non-renewable energy sources.
Keywords: NZEB (Net Zero Energy Building), N+NZEB (Nearly Net Zero Energy Building), REC (renewable energy certificates), wind turbines, Photovoltaic array.
Introduction
Over the past couples of decades, there has been a significant depletion of the fossil fuels in the world. Consequently, the energy sources have become more drained as opposed to the previous years. Following this observation, there have been increased concerns about the efforts to increase the provision of energy as well as the efficiency of the energy technologies. The studies have shown that the domestic energy consumption in the world is about 40%. Over the years, the renewable energy sources have been perceived as a vital alternative to the fossil fuels. The future holds a promising application of the building technology that generates electricity from the renewable resources harnessed from the buildings. In the future building, the buildings will be self-sustaining as far as energy is concerned. The new building referred to as the (NZEB) Net-Zero Energy Building the net energy in a year is zero. The annual net-zero energy means that the new buildings will produce exactly as much or more energy as its annual consumption. The energy application and production in the NZEB is associated with extensive research in the planning, prediction as well as efficiency of such building. In this paper, a grid-connectivity strategy for the NZEB has been outlined to enable the design optimization concerning the cost of the system as well as the availability of the resources (Erfan Saberbari, 2014).
Importance of NZEBs
Environmentalists all over the world have raised concern over the global warming as a result of the greenhouse effect. The buildings also contribute to the amount of carbon dioxide emitted into the atmosphere from the combustion of fossil fuels. The NZEB uses the green energy approach; the emission-free energy such as wind energy and solar are used in the provision of energy. The energy is produced within the NZEB hence the costs of transmission are limited. The NZEB technology is a significant strategy in a bid to lower the pollution as a result of the carbon dioxide emission.
The NZEB is a multifaceted value creation avenue. The NZEBs are expected to revolutionize the competitive advantages in the architecture as people approach towards the future of building technology. The value of properties such as land, industrial plant or residential building would improve with embracement of the NZEB technology. The market risks are supposed to be mitigated with the introduction of the NZEBs. For instance, the increased energy costs will be reduced when NZEBs are used. Moreover, since the NZEB utilize safe energy, the welfare and health of the inhabitants are promoted. For instance, the respiratory diseases from the smokes are eliminated in the NZEBs. The NZEB technology saves money; there is about 60% to 90% energy efficiency in the NZEBs. Using the NZEB architectural designs saves money over the whole life cycle of buildings; the energy maintenance costs are brought down. Moreover, the application of the NZEB technology is educational. The people who participate in design, construction and maintenance of an NZEB gain important knowledge in the energy management and energy efficiencies.
Drawbacks of NZEB
Although the concept of the NZEB seeks to reduce the average net energy per annum, it faces some drawbacks. There exist times when the NZEB will need the support of the grid connection to provide energy. The renewable energy sources have reliability problems; there are fluctuations in the capacity of the RES to produce adequate energy. When the RES become unreliable, the NZEB depends on the electricity grid whereby the efficiency of its components is compromised. Moreover, the efficiency of the NZEB depends on the behavior of the inhabitants. The energy management strategies depend on the energy consumption behaviors of the inhabitants. There are no hard set rules as to how energy is supposed to be used. There is the need to embrace the ways that can reduce the impact of these drawbacks.
Why NZEBs matter
The global consumption of energy in the world has been increasing. The 2013 survey by the IEO (International Energy Outlook) outlined a likely increase in the global energy consumption as indicated in figure 1.
Figure 1: Global energy consumption
The global energy consumption increases from 524 quadrillions Btu (2010) to a projected 630 quadrillion Btu (2020). Further projections show that in 2040, the global energy consumption will have hit 800 quadrillions Btu. The increase translates to 56% rise in every 30 years and demands for a strategic reduction in the energy consumption.
The fossil fuels contributed to most of the global energy consumption. The fossil fuels are one of the most harmful sources of energy. The global warming and consequent climatic changes are driven by the carbon dioxide emission related to the fossil fuels. The renewable energy sources contributed to 11% of the global energy source in 2013 (as seen in figure 2). However, the fossil fuels contributed to about 90% of the global energy sources. Because the fossil fuels are responsible for the increase in the global surface temperatures as a result of the greenhouse effect; there is a need to lower their use. The domestic energy consumption totaled to about 40%. Out of the 40% energy consumption, the fossil fuels contributed to about 30%; meaning that the emission of gases and the subsequent greenhouse effect was high.
Figure 2: Global Energy Sources in 2013
The energy consumption from the electricity in a typical building is about 70%. About 35% of the electricity used in the building is generated far away and then transmitted using wires. To curb the problems of high energy demand and transmission, the proposed future building (NZEB) will be designed to offer high energy efficiency. Moreover, the integration of the renewable sources will promote the energy generation in the NZEB. The NZEB targets to save about 40% energy because of the energy efficient design as well as utilization of the renewable energy sources. The propose NZEB design offers the production of energy that is later stored in the ESS (Energy Storage Systems) as indicated in figure 3.
Figure 3: The Architecture of the Electrical System for the new NZEB
The components of a typical NZEB
I) Energy Producers
The energy producers involve the mechanisms and devices used in the production of all the energies needed by the NZEB. The wind turbine and the photovoltaic cells are complementary sources of energy for an NZEB. In the summer, the photovoltaic cell which operates on solar energy becomes valuable. However, during the winter, the model functions on the wind generator. However, in the absence of both solar and wind, the electricity grid supplies the NZEB with energy.
A) Photovoltaic
The average capacity of a photovoltaic cell for use in a domestic house is about 4kW. The cost of a 4kW PV in the United States is about $6,000-$9,000. The following equation expresses the power output for a photovoltaic array;
Where stands for the solar radiation in (Kw/m2), is the effective surface area of the photovoltaic array in (m2), and represents the efficiency of the photovoltaic with relation to the power adapters. The average cost of a typical PV for the domestic NZEB is about $7000 due to maintenance costs for dusting and replacement of the stationary components.
Wind Generator
When the nominal wind speed is available (the wind speed between the highest and the lowest speed), the power output for a typical wind turbine is expressed by the following equation;
Where stands for the wind density (m/s), represents the sweep area in (m2), is the velocity of the wind in (m/s), refers to the power coefficient of the turbine and stands for the overall efficiency of the wind generator and the power conversion. The is frequently expressed as a function of the rotor speed ratio (TSR), and the theoretical value is 0.59. The is significantly influenced by the type of wind turbine employed. The size of the wind turbine and the mounting method used influences the value as well as the power output. The average energy output from a typical domestic wind turbine is about 6kW. The estimated cost of a domestic wind turbine (6kW) is between $20,000 and $30,000. The proposed NZEB is expected to use an average 3kW wind turbine that would cost roughly $12,000. If the turbine is well maintained, and well connected, it is expected to serve for 20 years, with a maintenance cost of $100 per annum.
C) Electricity Grid
The energy supply from the photovoltaic array and the wind turbine generator fluctuates with time. The electricity grid is needed to supply the necessary energy in the NZEB in case the renewable sources are insufficient. Moreover, the electricity grid is necessary for the distribution of the energy surplus from the NZEB. Therefore, the NZEB users benefit from the backup electricity that has zero emission.
II) Energy Consumers
Just as the other houses the NZEB also uses the home appliances that consume electricity such as the computers, lighting, and heating system.
A) Domestic Load
The energy consumption by the domestic lighting and the home appliances are classified as the household load. The inhabitants have a personal energy consumption due to the usage of the domestic lighting and appliances. An estimated domestic load is averaged at 18.1kWh daily for the NZEB.
B) PHEV
The PHEV refers to the rechargeable electronic devices. The studies have indicated that the PHEVs need to be charged during the of peak electric consumption scenarios. For instance, the electrical consumption at night is relatively low hence it is a favorable moment for charging the PHEVs. In the NZEBs, the PHEVs are charged using the stored energy and therefore, saves on the energy consumption. Most of the PHEVs ran on a lithium-ion battery and charged from a 240V outlet. The daily electrical consumption from the PHEVs is about 3.5kWh.
III) Energy Manager, Power Electronic Appliances, and Batteries
The energy from the renewable energy sources is unpredictable and erratic. The erratic nature of the RES affects the planning as well as the effectiveness of the NZEBs. The energy storage systems are significant parts of the NZEB because they promote availability and reliability of energy.
A) Batteries
The energy produced from the sporadic solar energy is stored in the batteries. The batteries operate as DC apparatus for the source of electricity. The batteries used in the NZEB needs to produce enough output for use in the NZEB. The standard battery for domestic energy should have a storage capacity of 6V or 2.76kWh. The following equation calculates the energy storage for a battery;
Where and represent the number of the functioning turbines, is the arrays, while refers to the total time that the turbines are providing the energy. The replacement costs for the batteries is estimated at $400 per year. The stationary nature of the batteries allows for the maintenance cost to be estimated as zero.
C) Power Electronic Appliances
Following the advanced technology in electronics, the power electronic devices can be applied as adaptors. There are also the smart devices fitted with microprocessors used for increasing the reliability of electric systems. In the NZEB model the electronic power devices include the rectifiers, inverters as well as adaptors.
D) Energy Manager
The energy produced, stored or used needs to be measured. The measurements of the energy are necessary for monitoring the efficiency, consumption rate as well as management. The energy management system in the NZEB is illustrated in figure 4.
Figure 4: Energy management system in an NZEB
IV) Environmental & Economic Factors
A) Solar Factor
There are different levels of solar energy across the world. There are several types of research conducted to determine the average global solar profile that offers the efficient as well as reliable energy. In the simulations, the average solar profile relevant for NZEBs is 2.125 kWh/m2/d. Solar energy is sporadic; however, the highest solar profile is estimated in June. Therefore, the maximum performance of the PV is highest during June.
B) Wind Factor
The annual wind reports obtained from different parts of the world indicate a heterogeneous pattern in the wind energy. The average wind profile for use in the NZEB has been studied. There has been a selected set of climates that give the effective speed and frequency of the wind. The reliability of the wind speed and strength determines the dependence of the NZEB on the electricity grid.
C) Price of Electricity
The NZEB is supposed to be connected to the electricity grid, for this reason, there is a cost incurred due to the use of the electricity from the grid. The annual cost of the electricity in a typical NZEB is estimated at $2,699. The cost of electricity depends on the reliability of the solar and wind energy. Moreover, the cost of electricity is affected by the maintenance of the NZEB components since during the maintenance, the energy from the RES may be temporarily unavailable.
Energy Management Strategies
Grid-Connection
The simulations involving the NZEB with three sources of energy has been proposed as the most efficient. In addition to the PV arrays and the Wind generators/turbines, the proposed NZEB has a third source of energy as the electricity grid. The architecture, as well as the cost of the NZEB design, is about $40,843 as indicated in Figure 6.
Figure 6: Architecture and cost of NZEB design
The proposed capacity of the wind turbine is 3kW, and the output power of the PV array is 8kW. The system is connected to a 4kW battery, and two rectifiers and inverter of 4kW each. Due to the maintenance costs, and the costs of electricity the annualized architectural design has an 18% increase, as shown in figure 7.
Figure 7: Annualized Architecture and cost of NZEB design
Although the RES is free and clean, numerous challenges affect their application. The reliability of the renewable sources of energy is a major challenge. There are fluctuations of solar isolation in different parts of the world. Moreover, the availability of the wind varies from one place to another. The amount of solar radiation and the speed of wind in a particular area also fluctuates throughout the year. As a result, the energy obtained from the PV arrays and the wind turbines also varies. It is, therefore, necessary to connect the NZEBs to an electricity grid to supplement the RES.
Building Niche Development
The government has a role in the establishment of the NZEB as the future of the building technology. Some researchers have proposed the application of the GAT (government assessment tool) in which the government sets the rules and regulations to restrict and motivate the adoption of the NZEB technology. The regulations on the building companies as well as the contractors to adopt the energy management and establishment of the NZEBs. The governance needed is expected to offer support in the organization of the building industry. Also, the subsidies on the materials needed for NZEBs can be provided. Restriction of the building technologies such as the elimination of the diesel generators to power buildings is significant. The government can uses strategies such as reduction of taxes on the renewable energy sources and NZEB components. Moreover, an increase in the tax for the non-renewable energy construction equipment is significant (Mansi Jain, 2017).
The N+NZEB (Net + Nearly Zero Energy Building)
The NZEB refers to a building whose annual total energy consumption is equal to the amount of energy it produces from the RES. In addition to the concept of NZEB, the N+NZEB refers to a high energy performance building. The energy used in the NZEB is produced from onsite renewable energy sources or nearby areas. The building consumes a reduced amount of energy and uses small amounts of RES. The energy consumption is computed on a monthly basis, the energy consumption and production throughout the year is almost equal. However, the monthly energy production and consumption may vary. The features of the N+NZEB are responsible for the following;
-Reduction of domestic energy consumption (cooling, heating, and lighting).
-Enhancement of the energy exploitation.
-Minimization of the auxiliary energy demands.
-Local production of the energy utilized in the building.
The following figure (Figure 8) outlines a typical N+NZEB architecture.
Figure 8: N+NZEB architectural model
In the model above, the strength of the building, as well as provision of critical elements for maintenance of energy are emphasized. The strategies of minimizing the energy consumption as well as maximizing the efficiency of the NZEB are included. The N+NZEB design has the following features;
I) High performance
The building has thermal insulation with the U-value of 1.0W/m2K. The light transmittance is averaged at 13% for the first floor and at 36% for the other floors. The building has a solar control of g=26%, and g=21% for the first floor and other floors respectively. The design has no thermal bridges.
II) Building features
The N+NZEB design is compact and has provisions for skylights.
III) Heat recovery
The building has energy recovery wheels for energy recovery. The global energy recovery for the wheels is at 70%. The waterside for the winter has a 900kW energy load from the industrial process. The lighting system includes full-LED (Light emitting diodes) with a lighting intensity power of 6.6W/m2. Use of the BMS maintains the LED lighting system. There are building inverters that vary the flow of the fan and the pump. The N+NZEB uses a PV of 585kW, thermal storage, and solar collectors.
All the factors mentioned above have been optimized to realize the interactions between the minimization of energy loses and total energy consumption. There exists a significant balance between the energy consumption during specific months and the amount of energy generated within these months. The energy balance is important in the process of designing the size of the NZEB as well as the size of the energy storage devices (Carlo Micono et al., 2015).
Use of the REC (Renewable Energy Certificates)
The RECs are provisions that can be bought as claims of compliance to renewable energy. Therefore the users voluntarily adhere to the application of the green technology. Within the building sector, the RECs can be applied as evidence for surpassing the building technology in which the NZEBs are emphasized. Once the person has satisfied the requirements for a ZEB, the REC can be given so that the person can track his/her use of the renewable energy. The broad use of the REC in the NZEBs creates an ‘N-ZEB community ‘whereby all the buildings use onsite generated renewable energy. The overdependence on the fossil fuels and other non-renewable energy are expected to lower significantly with the introduction of the renewable energy certificates (Kent et al., 2015).
Some countries such as the European states have formed monitoring policies as well as compliance partnerships that embrace the use of the NZEBs. In the future, the member states are expected to have only the NZEBs as the architectural designs. Several types of research have been conducted on the most effective designs and measurements for the efficient NZEB technology. The governments have supported the initiatives by creating the monitoring authorities that ensure that the NZEB technology is being implemented (Commission, 2017).
Automation of the NZEBs
The future of the building technology promised by the NZEBs depends on the application of the information technology. The ICT in the NZEBs is used for automation for energy management. The NZEBs require a well-designed automation that ensures that an equilibrium exists between the energy produced and the energy consumed. The automation is also important in the regulation of efficiency and energy losses. The automation in the context of the NZEB is relevant in the following;
a) Central automation of energy components
All the internal energy systems ought to be connected to ensure that the entire system functions with the highest efficiency. The central systems require automation to comply with the standards of the NZEB requirements in energy management.
b) Monitoring & feedback automation
Automated monitoring and feedback ensure that the low-energy consumption provided for by the NZEB is achieved. The automated monitoring and feedback system assist in following a precisely calculated climatic change establishment. Moreover, the automated monitoring promotes the saving of energy by the users. Saving energy forms a significant part of the NZEBs’ efficiency.
c) Automated load-shifting and the storage management
To increase the efficiency of the renewable energy produced onsite via the PV, automation is necessary for estimation of storage needs. The automated control of the components such as cooling and heating is necessary to avoid additional control loads. To avoid the unnecessary cooling, automation of the coolers at specific temperatures is necessary. Automation of the load-shifting also important in establishing the stability of the connection grid.
d) Automation for thermal comfort
The application of the highly efficient and low reacting NZEB systems such as the floor heating need automation. The automation can be incorporated in the weather forecasting to ensure that the thermal equipment function with high precision. The thermal equipment such as the concrete activators is highly efficient. However, their reaction time is lengthy hence automation is needed to improve their functionality.
In other words, the automation of the buildings in the NZEBs is important in the energy management and improvement of their efficiency. All the single components in the NZEBs need to be connected to the automation systems to improve the management of energy. The primary energy consumption will be highly reduced via automation. For instance, the automatic optimization operations will increase the renewable energy as well as monitor the efficiency of the NZEB at large. From the definition of what constitutes an NZEB, the automation offers sound criteria for realizing the goals of the NZEB concept. The concept of zero energy loss is empowered by the real-time reaction to the automation process. For instance, in an automated NZEB, both the systems and the NZEB building itself are automated. It means energy is consumed only when necessary, for instance; the room coolers and heaters only operate when certain temperatures are reached. It means that the automation of the thermal controllers works closely with the changes in the environment. The energy loses highly reduced, for instance, the heaters will not operate when it is hot. Unlike the manual operations, the energy consumption behaviors in the automated NZEBs is controlled. For instance, a security light will not work during the day in the automated scenario unlike in the manual operation whereby one may forget or neglect to switch it off.
Automation is aimed at improvement of the indoor climate. The automation takes the inhabitants’ actions without necessarily taking over the control. The automation influences the indoor climate by considering the external environment such as weather conditions.
The NZEB automation also minimizes the demand for energy. The availability of energy will greatly affect the prices of energy in the future. The market cost for electricity has variations due to the changes in the amount of production. The cost of electricity has been traditionally calculated following the low and high peak energy consumptions. Moreover, there is an increased company that provide reduced tariffs for peak loads such as the heat pumps. The increased application of the smart grid systems is expected to offer increased flexibility in the electricity tariffs. Automation of the NZEBs will ensure that the energy from the grid is only purchased when necessary. Having a controlled dependency on the grid increases the efficiency of the NZEBs as well as reduce the cost of energy. The automation of the NZEBs will incorporate the communication and feedback channels that will capture and store significant information. The users of an automated NZEB can keep track of the energy consumption behavior, changes in the energy demand and fluctuations in the energy costs. The effective monitoring of the energy consumption assists the users to make effective decisions concerning the energy management. For instance, the users can easily tell which month there was highest energy consumption. Moreover, the users can decide on what equipment to eliminate or improve in the NZEB system.
Although the energy mentioned above management strategies are crucial, the comfort of the inhabitants influences the energy consumption in an NZEB. Therefore, the energy usage behaviors are relevant in the energy management strategies. There is need to use the energy-saving technology in the NZEB including the low-energy consuming lighting, heating, and electronics. The energy consumption behaviors such as the use of hot water should only be applied when necessary. The energy regulations should comply with the dynamic energy requirements, changing energy consumption behaviors as well as the weather changes. For instance, there should be the application of the small-load wireless sensors to detect the environmental changes within the NZEBs. The wireless sensors are significant in the determination of the weather changes such as temperature changes.
Conclusion
The renewable energy sources such as the wind power and solar energy can be harnessed for use in the Net Zero Energy building (NZEB). The NZEB technology offers a promising future in architectures since it provides for the use of green energy as well as management of the energy consumption. However, the cost of the NZEB components, as well as maintenance of energy production, is quite high. The devices used in the productions, storage, and management of the energy in NZEBs are expensive. The utilization of the Renewable energy in the NZEB is challenged by fluctuations in the availability of solar and wind energy. The energy consumption behaviors of the inhabitants also influence the energy management in the NZEBs. For this reason, there has been numerous research conducted to improve the quality of energy as well as the reliability of the energy from the renewable sources. The use of the renewable energy has been emphasized due to the defects in the fossil fuels and other non-renewable energy sources. The fossil fuels are responsible for carbon dioxide emission that causes the greenhouse effect and the consequent global warming. Although some countries such as the European states have embraced the NZEB architecture, others have still not accepted to move to the NZEB technology. The NZEB offers the opportunity for the reduction of the dependency on the fossil fuels and hence contribute to a secure environment.
References
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