Pre-feasibility study of stand-alone hybrid energy systems for applications in eco-houses

Abstract

In light of rising cost of fossil fuels and fears of its depletion, coupled with the increase in energy demand and the rise in pollution levels, governments worldwide have had to look at alternative energy resources. Combining renewable energy generation like solar power with superior storage and conversion technology such as hydrogen storage, fuel cells and batteries offers a potential solution for a stand-alone power system. The aim of this paper was to assess the techno-economic feasibility of using a hybrid energy system with hydrogen fuel cell for application in an eco-house that will be built in Sultan Qaboos University, Muscat, Oman. Actual load data for a typical Omani house of a similar size as the eco-house was considered as the stand-alone load with an average energy consumption of 40 kW/day and 5 kW peak power demand. The National Renewable Energy Laboratory's Hybrid Optimisation Model for Electric Renewable software was used as a sizing and optimisation tool for the system. It was found that the total annual electrical energy production is 42,255 kW and the cost of energy for this hybrid system is 0.582 $/kW. During daylight time, when the solar radiation is high, the photovoltaics (PV) panels supplied most of the load requirements. Moreover, during the evening time the fuel cell mainly serves the house with the help of the batteries. The proposed system is capable of providing the required energy to the eco-house during the whole year using only the solar irradiance as the primary source.

Keywords:

• hybrid energy system
• eco-house
• fuel cell
• PV
• battery

1. Introduction

Energy is necessary for the development of mankind, which mainly depends on fossil fuel reserves. However, fossil fuel reserves are diminishing rapidly across the world with associated severe ecological and environmental consequences. A secure, benign, continuous and accessible energy supply is very essential for the sustainable development that meets present and future generations' needs. Hydrogen appears to be an environmentally clean energy carrier. It can be generated by electrolysis of water using energy generated by renewable sources. Using pure oxygen and hydrogen in fuel cells gives an output of electricity and water. Furthermore, if air is used instead of pure oxygen, then nitrogen oxide is produced (Momirlan and Veziroglu 1999, 2002, Beccali et al. 2008). Cotrell and Pratt (2003) have concluded that fuel cell systems supplied with electrolytic hydrogen produced by PV and wind turbine systems appear competitive in a 148 kW village power system if fuel cell prices are reduced by 40% of their capital cost. Santarelli and Macagno (2004) have considered PV-hydrogen system feeding an isolated residential building, they found that the energy cost could be competitive in the actual energy market if the cost of distribution power lines as well as the pollution cost are considered. Beccali et al. (2008) have shown that with the current costs of devices, fuels and electric energy purchased from the grid, the system that uses fuel cell supplied with natural gas and the system that uses electrolytic hydrogen with wind turbine are the most cost effective if the electricity rate increases by at least 100%.

One of the main requirements for a hybrid energy system is to ensure continuous power flow to the load. Excess energy from the renewable source could be stored so that it could be used when required. Although battery technology has reached a mature stage, however, the size, cost and disposal are still the constraining factors. Recent advancements in fuel cell and electrolyser technology have opened up the option for using a fuel cell as an energy storage device. A hybrid renewable energy system with fuel cell technology has been suggested by various researchers for small-scale power generation (Wallmark and Alvfors 2003, Santarelli et al. 2004, Kelouwani et al. 2005, Zoulias and Lymberopoulos 2007).

Oman has a high level of solar energy in all its regions, but this level is varying according to the location. The desert and northern parts of Oman areas have the highest solar energy density while the coastal areas in the southern part have the lowest solar energy density and relatively high wind speed (Al-Badi 2011a, 2011b, Al-Badi et al. 2011a, 2011b, 2011c, Al-Badi and Bourdosen 2011a, 2011b, Al-Yahyai et al. in press). An eco-house will be built at Sultan Qaboos University (SQU) in Muscat city in which the yearly average daily value of solar radiation is 6.4 kW/m²/day (Directorate General of Civil Aviation and Meteorology).

The electricity consumption in Oman during summer is about three times the consumption in winter. Most of energy consumption goes to building cooling. This sharp increase in energy requirement introduces heavy load on the economy and on the power generation sector in the country. The residential sector is the largest consumer category with its consumption taking more than half of the total system energy (The Authority for Electricity Regulation Oman 2010). Therefore, decreasing residential load can contribute effectively to reducing this huge power demand fluctuation. One way to achieve this is by making our buildings depend on natural, passive or active cooling methods instead of consuming more electric energy. Another way is to use renewable energy resources to provide the power requirements.

One of the research projects, which is conducted by researchers at SQU (Gastli et al. 2010), is designing and building of the first prototype of a typical Omani eco-house. The objective of this SQU eco-house project is to develop an innovative residential house that is besides being aesthetically pleasing and functionally working, economic, technically feasible, energy efficient, climatically appropriate and socio-culturally acceptable. The electric power for the eco-house will be supplied from a PV system. In case there is sufficient power from the sun, the PV system will deliver power directly to the loads in the house. If the energy delivered from the PV system is higher than the energy required by the house, then the excess of energy will charge batteries and/or produce hydrogen for storage or on site use. When the energy required by the load is higher, the PV system, the fuel cells and the batteries together will provide the energy for the load. The PV system, fuel cells, batteries charging system and load dispatching among the different sources of energy may be managed by a power management system.

This paper assesses the techno-economic feasibility of using hybrid energy system with hydrogen fuel cell for application in an isolated eco-house. Simulation results and sizing optimisation for the proposed system is presented. Moreover, analysis of power production and consumption of system are done. The analysis has been done on a representative day for 24 h in month of July when the peak power demand occurs. National Renewable Energy Laboratory's (NREL) Hybrid Optimisation Model for Electric Renewables (HOMERs version 2.68 beta) has been used to optimise the sizes of different hardware components in the PV-hydrogen system, taking into account the technical characteristics of system operation and minimising the total net present cost (NPC) of the system.

Analysis with HOMER requires information on resources, control methods, energy storage medium and economic constraints. Sensitivity analysis could be done with variables having a range of values instead of one number, which allows one to see the effects of change in a certain parameter on the overall system.

2. Hybrid energy system

A schematic diagram for the proposed hybrid system is presented in Figure 1. The proposed system consists of a photovoltaic module, a proton exchange membrane fuel cell stack, an electrolyser, batteries and converters. The fuel cell delivers the current difference between the load current and the PV current. If the PV generates more current than required by the load, then excess current is diverted towards the electrolyser.

Figure 1 Schematic diagram for the proposed system.

2.1 Solar energy resources radiation levels

The closest meteorological station for SQU campus is the station located at Muscat International Airport which is situated at around 10 km from SQU. The monthly average daily solar radiation for the period 2003–2007 for this site is depicted in Figure 2 (Ministry of Transport and Communications, Directorate General of Civil Aviation and Meteorology). The insolation levels for Muscat site are high and the yearly average daily values of solar radiation are more than 6.4 kW/m². The solar radiation during half of the summer season is the highest which coincides with the period of high electrical load demand as presented in Figure 2.

Figure 2 Monthly global radiation in Muscat (2003–2007).

2.2 Hybrid system components

2.2.1 Typical residential electrical load

For the residential consumer, the air conditioning is the largest energy use appliance followed by water heater, refrigeration, fans and lighting. The electrical load information for a typical house that has the same size as the eco-house is shown in Figure 3. The power consumption was calculated from the monthly electrical bills for this typical house for 2 consecutive years. The load increases during the summertime to almost three times compared with the winter season. An approximate hourly average weekly electricity consumption of a typical Muscat household in summer is shown in Figure 4.

Figure 3 Monthly average daily load for a typical house in Muscat that is the same size as the eco-house.

Figure 4 Hourly average weekly electricity consumption of a typical household in Oman during summer.

2.2.2 Batteries

The battery capacity and type were chosen from batteries specified in HOMER, Trojan L16P (6 V, 360 Ah, 2.16 kW) is considered in the model (Trojan Battery Company, Santa Fe Springs, CA USA). The cost of one battery is $300 (HYDROTURF International FZCO, Dubai, UAE) with a replacement cost of $280. The minimum lifetime for the battery is 5 years. Different number of batteries (2, 3, …, 9) were taken in the model.

The batteries are used to support the system during transients when the response of the system is not quick enough to meet the change in load demand. Batteries are needed when the household load is more than minimum fuel cell output, fuel cell is switched off to avoid wasting energy, or when fuel cell alone cannot respond instantaneously to step increases in household load or to provide for peak load.

2.2.3 Power converter

The function of the power converter is to maintain the flow of energy between the alternating current (AC) and direct current (DC) components. The initial cost for the converter was chosen to be $800/kW and operation and maintenance cost (O&M) of $5/kW with a lifetime of 15 years and efficiency of 90% (Retail Price List Inverters 2008, Ecodirect 2010). The replacement cost was taken as $800/kW. Three different sizes of converter (4.7, 5 and 5.5 kW) were taken in the model.

2.2.4 Economic and constraints

The lifetime of the projects is taken as 25 years, because the normal lifetime of photovoltaic cells ranges from 25–40 years (about Photovoltaic Cells, Bankier and Gale 2006). The annual discount rate is taken as 7.55%, which is the value used in Oman. A maximum annual capacity shortage is 7% and the operating reserve is 3%.

2.2.5 Fuel cell system

The cost of the fuel cell varies greatly depends on the type of technology used. It was reported by Retail Price List Inverters (2008) that the cost is around $4500/kW. However, it is expected that the cost will reduce in the future to about $400/kW (Khan and Iqbal 2005, Future Fuel Cells 2010, Fuel Cell Powering America). Considering these factors, the initial, replacement and operational costs are taken as $1000, $1000 and $0.02/h for the 1 kW system, respectively.

Several different sizes of fuel cells were considered in the calculation performed with HOMER: 2, 2.5, 3, 3.5, 4, 4.5, 5 and 5.5 kW. Fuel cell lifetime is considered to be 40,000 h (Fuel Cell Powering America).

2.2.6 Electrolyser

The cost of electrolysers is $1500–$3000/kW (Retail Price List Inverters 2008); according to water electrolyser manufacturers, mass production of these units is expected to result in at least a 50% cost reduction in the long term (Fuel Cell Powering America). In this paper, a 1 kW system is associated with an initial cost of $1000, replacement cost of $1000 and maintenance cost of $15/year. Different sizes of electrolysers (2.5, 5, 6, 7, 8, 9 and 10 kW) were investigated with HOMER. Lifetime is taken as 25 years with an efficiency of 75%.

2.2.7 Hydrogen tank

Hydrogen storage tanks are used to store the hydrogen produced from the electrolyser. The cost of 1 kg tank capacity is assumed to be $1000. The replacement and operational costs are considered as $1000 and $10/year (Khan and Iqbal 2005, Future Fuel Cells R&D 2010). Five different hydrogen storage tank options were considered in the optimisation process namely 5, 10, 15, 20 and 25 kg.

3. Discussion of results

One of the difficulties encountered in the design of stand-alone hybrid power system is the optimisation of different energy component sizes with respect to the cost of energy (COE) and overall system performance. HOMER was used to optimise the sizes of different components in the PV-fuel cell-battery hybrid system, taking into account the technical characteristics of system operation and minimising total NPC of the system. The initial cost of each component and other economic assumptions were important input to the software. These were identified from a search through available literature and direct contacts with hydrogen and renewable energy manufacturers as explained in previous sections. Different sizes of PV array, fuel cells, batteries, electrolyser and hydrogen tanks were taken in the search space in order to select the optimum system.

3.1 Economic analysis of the hybrid system

The project lifetime is taken as 25 years; the results of the economic analysis are shown in Table 1, where NPC (which is the present value of the costs of investment and operation of a system over its lifetime) for each component of the system are given. The PV panels and fuel cell system are the main cost factor of such a hybrid system. The table presents also the capital cost, the replacement cost, the O&M and the salvage cost (which is the residual value of power system components at the end of project lifetime) for each component. The total NPC, the capital cost and the COE for such a system are $101,644, $80,060 and 0.582 $/kWh, respectively.

Table 1 NPCs for the main components of the system.

Component	Size	Capital ($)	Replacement ($)	O&M ($)	Salvage ($)	Total ($)
PV	18 kW	39,600	12,347	2,301	− 6,920	47,328
Fuel cell	2 kW	2,000	1,900	2,299	− 88	6,112
Battery	9 Units (360 Ah)	2,700	3,656	575	− 145	6,786
Converter	4.7 kW	3,760	1,569	300	− 292	5,337
Electrolyser	10 kW	10,000	0	1,278	0	11,278
Hydrogen tank	22 kg	22,000	0	2,812	0	24,812
System		80,060	19,472	9,567	− 7,445	101,654

It should be mentioned that the COE is not competitive compared with non-renewable energy sources, owing to the high investment costs. However, considering that the system is a stand-alone one and allows the elimination of the costs of the distribution voltage lines as well as considering the cost due to pollution reduction would make the system more competitive in the market. Furthermore, increasing the price of energy together with a significant reduction of the investment cost and improvement in system efficiency will make such a system a competitive one.

3.2 Power production and consumption of PV-fuel cell system

All results related to the electricity production and load served are summarised in Table 2. As presented in the table, the optimal hydrogen-based system is operating 100% on renewable energy. The results of the simulation showed that the PV array produces 35.755 MW/year of electricity and the whole system had a total annual electrical energy production of 42.255 MW. Large amounts of energy (61%) are used to produce hydrogen and drive the fuel cell. The excess energy (12.5%), mainly resulted during wintertime, can be utilised for space heating. The percentage of capacity shortage of AC electrical load is 7% which occurs during the summertime, this can be overcome by either increasing the size of the power system or using power management system in the eco-house to disconnect the unnecessary load.

Table 2 Energy production and consumption for the PV-hydrogen system.

Annual energy production
35,755 kW (85%)	PV array
6499 kW (15%)	Fuel cell
100%	Renewable fraction
42,255 kW	Total production
Annual energy consumption
13,668 kW (39%)	AC primary load
21,531 kW (61%)	Electrolyser load
35,199 kW	Total load served
Other
5301 kW (12.5%)	Excess energy
1029kW (7%)	Capacity shortage

The analysis of the system showed that the fuel cell operated 4497 h/year and consumed approximately 390 kg of hydrogen per year. Its estimated lifetime was 8.89 years and its average electrical output was 1.45 kW. The monthly average electrical production from the PV panels and fuel cells is presented in Figure 5. Although the power demand is high in July (see Figure 3), the electric energy production from PV, during this time, decreases because of the low solar radiation (see Figure 2) and high temperature that reduces the efficiency of the PV panels.

Figure 5 Monthly average electric production.

3.3 Operational characteristic of the hybrid system

Figures 6 and 7 demonstrate the operational characteristics of the PV-battery-fuel cell system for a typical day of month of July. During the daytime, energy production and energy demand are not occurring at the same time, thus we need to store a large amount of energy and thereby the great usefulness of hydrogen to balance the system in an annual period. The eco-house load during the daytime is served almost by the PV panels, whereas the fuel cell and batteries take over the load during the nighttime. The noiseless operation of fuel cell during the night is another significant advantage compared with the use of diesel generator or wind turbine. It is obvious from Figure 6 that the power produced by the PV, during the daytime, is much greater than that needed by the load and as a result, the excess energy drives the electrolysis unit and is stored in the form of hydrogen gas, which supplies the fuel cell upon demand. Furthermore, part of this excess energy is stored in the battery unit as shown in Figure 7, thus during the summertime, all excess energy is utilised. Figure 8 shows that during the nighttime (1–7 am) part of the load is not met, in this case one solution is to increase the size of batteries, fuel cells or PV panels; however, this will increase the cost. A second solution is to have a power management system in the house that will keep only the necessary load energised and switches out other loads in the house. In the wintertime, the load requirement is met during 24 h and the excess energy can be utilised for space heating.

Figure 6 House load demand and power produced by PV and fuel cells for a typical day in July.

Figure 7 PV power, battery power and electrolyser unit consumption for a typical day in July.

Figure 8 House load demand, unmet load and power produced by PV, battery and fuel cells for a typical day in July.

4. Conclusions

The techno-economic aspects of using PV-fuel cell-battery for the eco-house in Oman were examined. Actual load data for a typical Omani house with similar size as eco-house were considered as the stand-alone load with an average energy consumption of 40 kW/day and 5 kW peak power demand. During daytime – when the solar radiation is high – the PV panels supplied most of the load requirements. Moreover, during the night hour the fuel cell mainly serves the house with the help of the batteries.

It was found that the COE for this hybrid system is 0.582 $/kW and the total annual electrical energy production is 42,255 kW. Large amounts of energy (61%) are used to produce hydrogen and drive the fuel cell. The excess energy (12.5%), mainly resulted during wintertime, can be utilised either for space heating. The COE is not competitive in the actual energy market which is due to the high investment costs. However, considering that the system is a stand-alone one and allows the elimination of the costs of the distribution voltage lines as well as considering the cost due to pollution reduction would make the system more competitive in the market.

The fuel cell operated 4497 h/year and consumed approximately 390 kg of hydrogen per year. Its estimated lifetime was 8.89 years and its average electrical output was 1.45 kW. The proposed system is capable of providing the required energy to the eco-house during the whole year. It is recommended to use power management system to manage the PV system, fuel cells, batteries charging system and load dispatching in the eco-house.