End-use energy utilization efficiency of Nigerian residential sector

Abstract

In this paper, the end-use efficiencies of the different energy carriers and the overall energy efficiency in the Nigerian residential sector (NRS) were estimated using energy and exergy analysis. The energy and exergy flows were considered from 2006 to 2011. The overall energy efficiency ranges from 19.15% in 2006 to 20.19% in 2011 with a mean of (19.96±0.23)% while the overall exergy efficiency ranges from 4.34% in 2006 to 4.40% in 2011 with a mean of (4.31±0.059)%. The energy and exergy efficiency margin was 15.58% with a marginal improvement of 0.07% and 0.02%, respectively when compared with previous results. The contribution of the energy carriers to the total energy and exergy inputs were 1.45% and 1.43% for electricity, 1.95% and 3% for fossil fuel and 96.6% and 95.57% for bio-fuel. The result shows that approximately 65% of the residence use wood and biomass for domestic cooking and heating, and only a fraction of the residence have access to electricity. LPG was found to be the most efficient while kerosene, charcoal, wood and other biomass the least in this order. Electricity utilization exergy efficiency is affected by vapor-compression air conditioning application apart from low potential energy applications. In addition, this paper has suggested alternatives in the end-use application and has demonstrated the relevance of exergy analysis in enhancing sustainable energy policies and management and improved integration techniques.

Graphical abstract

Keywords

end-use / energy / exergy efficiency / residential sector / Nigeria

1 Introduction

Conventional energy resources in the world are rapidly depleting, and the need for efficient utilization of energy resources is becoming a leading global issue that is gaining significant attention. Based on this reason, related issues on energy conversion processes, energy conservation and efficient energy utilization in both industries and all final energy consuming sectors are of immense importance to energy engineers, scientists, administrators and policy makers [1,2].

In the past three decades, Nigeria has witnessed an increase in energy consumption especially in the residential sector. From 1980 to 2008 the energy consumption has increased steadily at an annual rate of 9.2%, followed by a decrease of 5.4% in 2010 and then an increase of 20.5% in 2011 [3]. Nigeria’s expectation is to be among the 20th industrialized nations of the world by 2020. By this projection, it is expected that the energy consumption will surely increase in all the sectors due to expected increase in economic activities. The energy production and consumption sectors are faced with numerous challenges including low energy generation, low energy efficiency, high losses in conversion processes, poor management systems and high environmental degradation and emissions [4]. For Nigeria to make a meaningful progress in her energy crusade, the country will need to address the issues of low energy efficiency, ensure energy optimization in all sectors of the economy, improve environmental and social security and initiate a proper energy management system to meet the requirements for sustainable development. The end-use energy utilization and the overall efficiency of Nigerian residential sector are described in terms of exergy in this paper. This employs both the mass and energy conversion principles with the second law of thermodynamics. Exergy analysis is a thermodynamic tool for locating and quantifying losses in thermal systems and processes. It is also capable of differentiating levels of energy sources (high-quality and low-quality) and their degree of utilization in different sectors, by providing a platform to equate quality energy sources with high temperature applications for better efficiencies [5]. Consequently, exergy analysis can be applied effectively in handling the impact due to energy resource consumption on the immediate environment and as a strategy for achieving the objectives of effective energy-resource utilization [6,7].

Many studies have been conducted on energy and exergy utilization efficiencies of different sectors [8–17]. In recent years, exergy analysis has become an important management tool required for economic utilization of energy resources in any energy consuming sector. This accounts for the past numerous and extensive work published on energy and exergy utilization efficiencies of different sectors with the aim of improving the performance of energy conversion systems. For example, the transportation sector [18–20], industrial sector [1,21], commercial sector [11,22] and agricultural sector [23,24]. Although, extensive literature exist concerning energy utilization efficiencies of residential sectors of several countries as contained in Refs. [11,22,25]. There have not been many records for Nigeria except what was reported Badmus and Osunleke in Ref. [26]. Their results show that the mean energy and exergy efficiencies of Nigerian residential sector (NRS) from 1991 to 2005 was 19.89 and 4.38%, respectively, which was considered low when compared to other countries in the world. In addition, in 2005 the Nigerian government launched an expanded energy program geared toward improving energy efficiency in the residential sector [27]. Since the launch of this program no work has been published in the open literature on the performance of the residential sector. For sustainable development, there is a need for regular energy audit in various sectors of the economy. This will provide adequate information on the status of energy flow and utilization, thereby enhancing effective planning. The objectives of this paper, therefore, are to evaluate the end-use efficiencies of the different energy carriers used in the sector and the overall energy utilization efficiency of the sector from 2006 to 2011 using energy and exergy application, provide a complete breakdown of the total energy and exergy flows and losses by each energy carrier in the end-use as well as suggest measures for efficiency improvement in the sector. The findings in this paper are believed will provide a platform for informed decision on sustainable energy policy by the government.

2 Methodology and data sources

The data used for analysis were extracted from the achieves of the National Bureau of Statistics and National Population Commission of Nigerian report [3,28]. For simplicity, the energy carries were classified as electrical energy (energy from electricity), fossil fuel energy (LPG, kerosene and charcoal) and bio-fuel energy (wood and other biomass). The energy and exergy efficiencies, product and reference temperature of different types of electrical appliances, energy carriers and end-use applications as applied to this paper are contained in Refs. [17,29]. The energy and exergy models were then applied to the energy data and the parameters of interest were all determined.

The general energy and exergy balance equation for unsteady-flow process are expressed in Ref. [5] as

(1){\displaystyle \sum _{in}\left(m_{in}\theta \right){}_{in}-{\displaystyle \sum _{ex}\left(m_{ex}\theta \right){}_{ex}+{\displaystyle \sum _{b}Q{}_b-W=\mathrm{0}}},}

(2){\displaystyle \sum _{in}{ϵ}_{in}}{m}_{in}\theta -{\displaystyle \sum _{ex}{ϵ}_{ex}}{m}_{ex}\theta +{\displaystyle \sum _b{E}^Q}-E{}^W-I=\mathrm{0},

(3)\theta =h+ke+pe,

where h,ke \mathrm{and} pe are specific enthalpy, kinetic and potential energy respectively, m is the mass flow rate, ϵ is specific exergy, Q_b is the heat transfer across the system boundary b, W is the work, and I is the exergy consumption. The change of exergy of a system can either be positive or negative during a process, but the exergy consumed cannot be negative. The decrease of exergy principle can be summarized as

(4)

For a closed system, Eqs. (1) and (2) are simplified as

(5){\displaystyle \sum _{b}Q{}_b-W=\mathrm{0}},

(6){\displaystyle \sum _b{E}^Q}-E{}^W-I=\mathrm{0.}

The components of exergy are expressed mathematically in sections 2.1 to 2.3.

2.1 Chemical exergy

Chemical exergy is the highest amount of work obtained when the system approaches the state of equilibrium with the environment by heat transfer processes involving the exchange of materials only with the environment [17,30]. The most common mass flow or energy carriers are hydrocarbon/fossil/biomass fuels, whose specific exergy can be expressed as

(7)ϵ{}_{ff}={\mathit{\gamma }}_{ff}{H}_{ff},

where {\mathit{\gamma }}_{ff} is the fuel exergy grade function, defined as the ratio of fuel chemical exergy to the higher heating value, H_{ff} of the fuel . The values of H_{ff}, {ϵ}_{ff} and {\gamma }_{ff} for some fuels considered in this paper are presented in Table 1 [17].

2.2 Reference environment

Exergy is mostly evaluated with respect to a reference temperature (environment). The reference environment is in a dead state and experiences only internal reversible processes in which the intensive properties (temperature T_{\mathrm{0}}, pressure P_{\mathrm{0}} and chemical potentials {\mu }_{\mathrm{00}}) of the environment remains constant. Based on the weather condition in Nigeria (a tropical Sub-Saharan West African country, which lies between latitudes 4° and 13° 9′ North of the Equator and longitudes 2° 2' and 14° 30′ East) and coupled with slight modifications of the Gaggioli and Petit’s model that was further recommended by Dincer et. al [17], this paper assumed a reference temperature of T₀ = 27°C and a pressure of P₀ = 100 kPa as surrounding pressure. While saturated air with water vapor and the following condensed phases are considered at 27°C and 100 kPa: water (H₂O), gypsum (CaSO₄⋅2H₂O) and limestone (CaCO₃) [31].

2.3 Energy and exergy efficiencies for principal types of processes

The energy (\eta) and exergy (\psi) efficiencies for the considered processes in this paper are expressed in Eqs. (8) and (9).

(8)\eta ={\displaystyle \frac{\mathrm{Energy} \mathrm{in} \mathrm{products}}{\mathrm{Total} \mathrm{energy} \mathrm{input}}},

(9)\psi ={\displaystyle \frac{\mathrm{Exergy} \mathrm{in} \mathrm{products}}{\mathrm{Total} \mathrm{exergy} \mathrm{input}}}.

The exergy efficiencies can be written as a function of corresponding energy efficiencies by assuming the energy grade function to be unity [17] which is valid for the fuels used in this paper. The energy and exergy efficiencies for various principal types of processes are presented in sections 2.3.1 and 2.3.2.

2.3.1 Electric heating

Processes involving electric and fossil fuel heating, generate product heat Q_p at a constant temperature T_p, either from electrical W_e or fuel mass m_ff. The energy and exergy efficiencies for electrical heating are expressed [17]:

(10){\eta }_{\mathrm{h,}e}={\displaystyle \frac{Q_{\mathrm{p}}}{W_e}},

(11){\psi }_{\mathrm{h,}e}={\displaystyle \frac{E^{Q_{\mathrm{p}}}}{E^{W_e}}}=\left(\mathrm{1}-{\displaystyle \frac{T_{\mathrm{0}}}{T_{\mathrm{p}}}}\right){\displaystyle \frac{Q_{\mathrm{p}}}{W_e}}\cong \left(\mathrm{1}-{\displaystyle \frac{T_{\mathrm{0}}}{T_{\mathrm{p}}}}\right){\eta }_{\mathrm{h,}e},

where {\eta }_{\mathrm{h,}e} and {\psi }_{\mathrm{h,}e} are energy and exergy efficiencies for electrical heating, T_{\mathrm{p}} is product temperature, T_{\mathrm{0}} is environmental temperature, Q_{\mathrm{p}} is product heat and W_e is electric work production. For fuel heating the energy and exergy efficiencies are:

(12){\eta }_{\mathrm{h,}ff}={\displaystyle \frac{Q_{\mathrm{p}}}{m_{ff}}}{H}_{ff},

(13){\eta }_{\mathrm{h,}ff}={\displaystyle \frac{Q_{\mathrm{p}}}{m_{ff}}}{H}_{ff},

2.3.2 Electric cooling

The energy and exergy efficiencies for electric cooling are expressed as

(14){\eta }_{\mathrm{c,}e}={\displaystyle \frac{Q_{\mathrm{p}}}{W_e}},

(15){\psi }_{\mathrm{c,}e}={\displaystyle \frac{E^{Q_{\mathrm{p}}}}{E^{W_e}}}=\left(\mathrm{1}-{\displaystyle \frac{T_{\mathrm{0}}}{T_{\mathrm{P}}}}\right){\displaystyle \frac{Q_{\mathrm{p}}}{W_e}}=\left(\mathrm{1}-{\displaystyle \frac{T_{\mathrm{0}}}{T_{\mathrm{P}}}}\right){\eta }_{\mathrm{c,}e}.

2.3.3 Electric generation

The energy and exergy efficiencies for electricity generation through fossil fuels are

(16){\eta }_{e,ff}={\displaystyle \frac{Q_e}{m_{ff}{H}_{ff}}},

(17)y_{e,ff}={\displaystyle \frac{E^{W_e}}{m_{ff}}}{e}_{ff}={\displaystyle \frac{W_e}{m_{ff}{g}_{ff}{H}_{ff}}}\cong {\eta }_{e,ff}.

2.4 Weighted mean energy and exergy efficiencies

The weighted mean energy and exergy efficiencies are evaluated for the residential sector using the following steps.

1) Weighted mean electrical energy and exergy efficiencies

Weighted mean for electrical energy and exergy is calculated using the weighting factor which is expressed as the ratio of the electrical energy input for the electrical appliance to the electrical energy input by all the appliances. The expressions for energy and exergy weighted means are

(18)R_{\eta e}={\displaystyle \frac{\left[\left(e_{\mathrm{AC}}\times {\eta }_{\mathrm{AC}}\right)+\left(e_{\mathrm{lighting}}\times {\eta }_{\mathrm{lighting}}\right)+\left(e_{\mathrm{cooking}}\times {\eta }_{\mathrm{cooking}}\right)+\left(e_{\mathrm{appliances}}\times {\eta }_{\mathrm{appliances}}\right)\right]}{\left(e_{\mathrm{AC}}+e_{\mathrm{lighting}}+e_{\mathrm{cooking}}+e_{\mathrm{appliances}}\right)}},

(19)R_{\psi e}={\displaystyle \frac{\left[\left(e_{\mathrm{AC}}\times {\psi }_{\mathrm{AC}}\right)+\left(e_{\mathrm{lighting}}\times {\psi }_{\mathrm{lighting}}\right)+\left(e_{\mathrm{cooking}}\times {\psi }_{\mathrm{cooking}}\right)+\left(e_{\mathrm{appliances}}\times {\psi }_{\mathrm{appliances}}\right)\right]}{\left(e_{\mathrm{AC}}+e_{\mathrm{lighting}}+e_{\mathrm{cooking}}+e_{\mathrm{appliances}}\right)}},

where R_{\eta e} and R_{\psi e} are the weighted mean energy and exergy efficiencies while e_{\mathrm{appliances}}, {\eta }_{\mathrm{appliances}} and {\psi }_{\mathrm{appliances}} are the appliances energy consumption, energy efficiency and exergy efficiency, respectively.

2) Weighted mean for fossil-fuel energy and exergy efficiencies

(20)R_{\eta ff}={\displaystyle \frac{\left(e_{\mathrm{cooking}}\mathrm{LPG}\times {\eta }_{\mathrm{cooking}}\mathrm{LPG}\right)+\left(e_{\mathrm{cooking}}\ker \mathrm{o}\times {\eta }_{\mathrm{cooking}}\mathrm{kero}\right)+\left(e_{\mathrm{heating}}\mathrm{kero}\times {\eta }_{\mathrm{heating}}\mathrm{kero}\right)}{e_{\mathrm{cooking}}\mathrm{LPG}+e_{\mathrm{cooking}}\mathrm{kero}+e_{\mathrm{heating}}\mathrm{LPG}+e_{\mathrm{heating}}\mathrm{kero}}},

(21)R_{\psi ff}={\displaystyle \frac{\left(e_{\mathrm{cooking}}\mathrm{LPG}\times {\psi }_{\mathrm{cooking}}\mathrm{LPG}\right)+\left(e_{\mathrm{cooking}}\ker \mathrm{o}\times {\psi }_{\mathrm{cooking}}\mathrm{kero}\right)+\left(e_{\mathrm{heating}}\mathrm{kero}\times {\psi }_{\mathrm{heating}}\mathrm{kero}\right)}{e_{\mathrm{cooking}}\mathrm{LPG}+e_{\mathrm{cooking}}\mathrm{kero}+e_{\mathrm{heating}}\mathrm{LPG}+e_{\mathrm{heating}}\mathrm{kero}}}.

3) Weighted mean bio-fuel energy and exergy efficiencies

(22)R_{\eta bf}={\displaystyle \frac{\left(e_{\mathrm{cooking}}\mathrm{wood}\times {\eta }_{\mathrm{cooking}}\mathrm{wood}\right)+\left(e_{\mathrm{heating}}\mathrm{wood}\times {\eta }_{\mathrm{heating}}\mathrm{wood}\right)+\left(e_{\mathrm{cooking}} \mathrm{other} \mathrm{biomass}\times {\eta }_{\mathrm{cooking}} \mathrm{other} \mathrm{biomass}\right)}{e_{\mathrm{cooking}}\mathrm{wood}+e_{\mathrm{heating}}\mathrm{wood}+e_{\mathrm{cooking}} \mathrm{other} \mathrm{biomass}}},

(23)R_{\psi bf}={\displaystyle \frac{\left(e_{\mathrm{cooking}}\mathrm{wood}\times {\psi }_{\mathrm{cooking}}\mathrm{wood}\right)+\left(e_{\mathrm{heating}}\mathrm{wood}\times {\psi }_{\mathrm{heating}}\mathrm{wood}\right)+\left(e_{\mathrm{cooking}}\mathrm{other} \mathrm{biomass}\times {\psi }_{\mathrm{cooking}}\mathrm{other} \mathrm{biomass}\right)}{e_{\mathrm{cooking}}\mathrm{wood}+e_{\mathrm{heating}}\mathrm{wood}+e_{\mathrm{cooking}}\mathrm{other} \mathrm{biomass}}}.

4) Overall weighted mean energy and exergy efficiencies

The overall weighted mean for energy and exergy efficiencies are obtained first by determining the weighting factors as expressed for the different energy carriers in Eqs. (24) to (26).

(24)W{F}_e={\displaystyle \frac{{\displaystyle \sum e}}{{\displaystyle \sum e}+{\displaystyle \sum ff+{\displaystyle \sum bf}}}},

(25)W{F}_{ff}={\displaystyle \frac{{\displaystyle \sum ff}}{{\displaystyle \sum ff}+{\displaystyle \sum e+}{\displaystyle \sum bf}}},

(26)W{F}_{bf}={\displaystyle \frac{{\displaystyle \sum bf}}{{\displaystyle \sum bf+{\displaystyle \sum e}}+{\displaystyle \sum ff}}},

where W{F}_e, W{F}_{ff} and W{F}_{bf} are the weighting factors for energy and exergy of electrical, fossil and bio-fuel, respectively, and {\displaystyle \sum e}, {\displaystyle \sum ff} and {\displaystyle \sum bf} are the overall electrical, fossil and bio-fuel energy consumption. The overall weighted energy and exergy efficiencies are presented in Eqs. (27) and (28).

(27)$$\begin{gathered} R_{{\eta }_{\mathrm{O}}}=\left(R_{{\eta }_e}+W{F}_e\right)+\left(R_{{\eta }_{ff}}+W{F}_{ff}\right) \\ +\left(R_{{\eta }_e}+W{F}_e\right), \end{gathered}$$

(28)$$\begin{gathered} R_{{\psi }_{\mathrm{O}}}=\left(R_{{\psi }_e}+W{F}_e\right)+\left(R_{{\psi }_{ff}}+W{F}_{ff}\right) \\ +\left(R_{{\psi }_{ff}}+W{F}_{bf}\right). \end{gathered}$$

3 Results and discussion

The mean energy and exergy efficiencies were determined for each year and for each source of energy carrier applying the conversion efficiencies for the respective carriers. The overall energy and exergy efficiencies range from 19.15% in 2006 to 20.19% in 2011 with a mean of (\mathrm{19.96}\pm \mathrm{0.23})\mathrm{\%} and 4.34% in 2006 to 4.44% in 2011 with a mean of (\mathrm{4.31}\pm \mathrm{0.059})\mathrm{\%}, respectively. Their margin is approximately 15.58%, which is the difference between energy and exergy efficiency. This margin is considered to be too wide, and the reason may be attributed to mismatch existing between input and output quality levels of energy, which means high temperature energy resources were utilized for relatively low temperature applications.

However, comparing energy and exergy efficiencies of previous study obtained from 1991 to 2005 [26] and those with the present study the improvement is marginal approximately 0.07% and 0.02% for energy and exergy efficiency, respectively. The energy and exergy efficiencies for the residential sector of some countries are presented in Table 2 [12,16,26,31–33] with Nigeria having the lowest value for energy efficiency while exergy efficiency is only higher than those of Japan and Italy. The result indicates that the NRS is still inefficient and requires a review of the existing measures.

3.1 Energy and exergy product losses and flow distribution

Figures 1 to 3 show the yearly trend of energy and exergy product losses for different energy carriers from 2006 to 2011, which reveals that the sector is experiencing large losses. The energy and exergy losses are found to be high in bio-fuel (wood and biomass) followed by fossil fuel with electrical energy having fewer losses. The small losses in electrical energy consumption may be attributed to the policy change of 2005 and 2006 in the sector. Figures 4 and 5 demonstrate the energy and exergy flow diagrams in terms of energy and exergy inputs, products and losses for the year 2011. The overall energy and exergy inputs are 2937422 TJ and 2969167 TJ, respectively while the share of energy and exergy inputs are 42435 TJ, 57365 TJ, 2837622 TJ and 42435 TJ, 89109 TJ, 2837623 TJ for electrical, fossil fuel and bio-fuel energy carriers, respectively.

3.2 Analysis of end-uses and contribution of energy carriers

3.2.1 Electrical energy contribution

Electrical energy source is widely used source of energy in Nigeria as observed from this paper. The various end-uses include air-condition (A/C), lighting, cooking, water heating and powering of domestic appliances like television, fans and VCD/DVD player. The end-use energy efficiencies vary from 5.85% to 79.6% with lighting having the least efficiency of 5.85%. From Fig. 4, it has been observed that lighting and cooking consumed approximately 23.47% and 32.2% of the total household electrical energy input in 2011. Air-conditioning, water heating and household appliances consumed 14.62%, 12.81% and 16.89%, respectively.

From Fig. 5, it is observed that the total electrical exergy inputs accounts for 42435 TJ from which 28.55% losses are caused by cooking (use of electric stove) and 22.53% losses result from the use of household appliances. Lighting and heating contribute approximately 10.48% and 19.99% to the total losses, respectively. The mean energy and exergy efficiencies for electrical energy are found to be 42.20% and 9.62%, respectively. The end-use exergy efficiencies vary from 4.1% to 18.9% with lighting having the least value of 4.1%. However, the overall energy and exergy values for electrical energy in 2011 are 20.19% and 4.40%, respectively. The poor electrical exergetic performance observed is largely due to its application in lighting with incandescent bulbs, vapor compression refrigerated air-conditioning, cooking and water heating. This can be replaced with systems like vapor absorption air-conditioning, lighting with low energy bulbs, replacing cooking and heating with the use of LPG and solar heating, which will improve the end-use utilization efficiency and thus the overall efficiency.

3.2.2 Fossil fuel energy contribution

The following fuels were considered under fossil fuel, LPG, kerosene and charcoal. The end-use of LPG in the sector is for cooking and water heating, the end-use of kerosene is for cooking, water heating and lighting, and the end-use of charcoal is for cooking only. The end-use energy efficiencies vary from 4.1% to 73.2% with kerosene having the least lighting efficiency of 4.1% and LPG having the highest utilization efficiency of 73.2%. From Fig. 4, it is noticed that lighting, cooking and heating with kerosene consume approximately 6.47%, 40.65% and 9.19%, respectively of the household energy of the total fossil fuel contribution in 2011 while cooking and heating with LPG and charcoal consume approximately 1.2%, 0.19% and 40.19%, respectively.

The total exergy inputs for fossil fuel was 57365 TJ in 2011 (Fig. 5), from which 41.4% of losses were caused by lighting with kerosene, 24.99% by cooking and 6.04% by heating. Similarly, cooking and heating with LPG contributes 0.69% and 0.12%, respectively to the total exergy losses while cooking with charcoal contributes 26.71%. The end-use exergy efficiencies vary from 1.45 to 14.49 % with kerosene having the least lighting efficiency in its end use and LPG having a cooking efficiency of 14.49%. The gap between end-use energy and exergy efficiencies is very large. In all, the mean energy and exergy efficiencies of LPG, kerosene and charcoal in 2011 were, 62.62%, 13.46%, 17.34%, and 26.22%, 6.26%, 3.60%, respectively.

Kerosene and charcoal are found to have the least energy and exergy efficiencies among the fossil fuels while charcoal is found to be the best used energy source. A careful observation indicates that the main factor responsible for the low efficiency utilization of kerosene in the sector is its application for lighting purposes, evident in the large exergy losses. Comparing cooking efficiencies only, the result reveals that LPG and kerosene have better cooking efficiencies than charcoal. The cooking and heating efficiencies of kerosene can be enhanced by design of improved kerosene stoves for cooking and the use of thermal solar collectors for water heating instead of kerosene and LPG will improve the overall efficiency.

3.2.3 Bio-fuel energy contribution

The two fuels considered under bio-fuel energy are wood-fuel and other biomass. Wood-fuel is used for both cooking and heating while other biomass is used for cooking only. In Nigeria, these fuels are the mostly used since approximately 65% of the population are rural dwellers. The end-use energy efficiencies vary from 5.67% to 11.5%. The result shows that the end-use efficiency of wood-fuel for heating is higher than that for cooking. This difference is based on the system used for energy conversion. From Fig. 4, it is observed the total energy input from bio-fuel is 2837622 TJ from which 9.8% and 28.35% of the total energy contributed by wood-fuel is used for heating and cooking, respectively while 61.84% of the energy is contributed by other biomass and used for cooking only. The mean energy and exergy efficiencies of wood-fuel and other biomass are 21.28%, 18.60% and 4.39%, 4.15%, respectively. From Fig. 5, it is seen that approximately 12.22% and 34.52% of exergy losses are caused by heating and cooking with wood while 75.56% of losses resulted from cooking with biomass. The end-use exergy utilization efficiencies vary from 2.48% to 4.73%. The end-use efficiencies of wood-fuel and other biomass fuel are the lowest compared with all the fuels studied in this paper. For biomass, the poor combustion performance is obvious, since most of these wastes are used in open fires or domestic tripods. In this circumstances, the energy produced by combustion of the wastes is transferred away from where it is highly needed for cooking. Moreover, to improve the combustion efficiency of the agricultural wastes, an additional processing is required either by converting them into briquettes for effective utilization as an energy carrier. The need for improved woodstoves is also be necessary to enhance the performance efficiency of cooking and reduction in CO₂ contribution.

4 Overview of the residential sector

Lighting alone accounts for approximately 18% of the total power consumed in the residential sector compared to approximately 8%−11% in advanced countries. Over 60% of Nigerians are still using incandescent bulbs for their domestic lighting. Nigeria needs approximately 50 million compact fluorescent lamps (CFL) to replace the incandescent lamps in the residential sector [3]. With this replacement, a saving of approximately 1500 MW of electricity can be achieved [34]. The result in this paper shows that lighting consumed approximately 23.47% of the total electrical energy in 2011 with 10.48% losses due to lighting. The use of CFL’s in the sector will be a measure of efficiency improvement.

Furthermore, in 2011 the total amount of kerosene consumed in NRS was approximately 8.77×10¹³ kJ and 577×10⁴ tons of CO₂ was emitted due to the consumption of kerosene [35]. The research shows that most urban dwellers use kerosene for cooking, heating and lighting. The end-use efficiency of kerosene is low largely due to its utilization in lanterns. Similarly, the contribution of biomass to the total energy input was over 90%, meaning that most Nigerians are rural dwellers and use biomass mostly as their major source of domestic energy for cooking. The energy and exergy efficiencies of biomass were 4.39% and 4.15%. These small values were responsible for low overall energy and exergy efficiencies of the sector.

The biogas potential of Nigeria need to be developed to replace biomass for rural utilization in cooking and heating. The current biomass can be effectively utilized if properly treated or converted into briquettes and used in improved stoves. Nigeria has a large amount of proven gas reserves. The production capacity in 2011 was approximately 39.9 billion cubic meters (about 1.2% of the world share) and fled approximately 2.5 billion cubic feet per day (bcf/d) [36]. It is necessary to develop the gas industry for effective domestic and household consumption. However, solar thermal application should be considered for water heating. The country has an annual average sunshine of approximately 6.25 h. The average daily solar radiation is about 5.25 kW/(m²·d), ranging from 3.5−7.0 kW/(m²·d) [37]. With the replacement of heating by the solar system, the availability of fossil fuel will be increased since only a fraction is now utilized for water heating [26]. The main factor that affects the overall efficiency of the sector is end-use applications of energy resources. If end-use applications are utilized by appropriate energy carriers, the perfor-mance of the sector will increase with reduced losses.

5 Conclusions

The end-use energy utilization efficiencies, overall energy and exergy efficiencies of the residential sector of Nigeria were evaluated. The main conclusions are:

1) The energy and exergy efficiencies in Nigerian residential sector have improved marginally from 2006 to 2011 when compared with the values from 1991 to 2005.

2) The overall energy efficiency ranges from 19.15% in 2006 to 20.19% in 2011 with a mean of (\mathrm{19.96}\pm \mathrm{0.23})\mathrm{\%} while the overall exergy efficiency ranges from 4.34% in 2006 to 4.44% in 2011 with a mean of (\mathrm{4.31}\pm \mathrm{0.059})\mathrm{\%} and margin of 15.58%.

3) Electricity has been found to be the widely used energy source, especially in the urban areas during the period of the study. The mean energy and exergy values are 42.76% and 13.77%, respectively. The end-use energy and exergy efficiencies vary from 5.85% to 79.9% and 4.1% to 18.9%. The poor end-use performance resultes from its application in air-conditioning, lighting with incandescent bulbs, cooking and water heating.

4) Fossil fuel: The most efficient energy source is LPG, used for heating and cooking. The mean energy and exergy efficiencies are 62.63% and 13.46%, respectively. The end-use efficiencies vary from 4.1% to 73.2% for energy and 0.45% to 14.49% for exergy. The poor utilization efficiency resulted, because of its use for lighting and cooking purposes.

5) Bio-fuel: The mean energy and exergy efficiencies of wood-fuel and other biomass are 21.28%, 4.15% and 18.60%, 4.39%, respectively. Approximately 12.22% and 34.52% of exergy losses are caused by heating and cooking with wood-fuel, respectively, while 75.56% of losses are caused by cooking with other biomass. The end-use utilization efficiencies of the two fuels are low due to its application in heating and cooking.

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