STUDY OF DIFFERENT MICRO CHP ALTERNATIVES FOR RESIDENTIAL APPLICATION Hongbo Ren 1†, Weijun Gao 1, and Yingjun Ruan 2 1

Faculty of Environmental Engineering, The University of Kitakyushu, Kitakyushu, Japan 2 Faculty of Human-Environment Engineering, Kyushu University, Fukuoka, Japan

ABSTRACT The growing worldwide demand for less polluting forms of energy has led to a renewed interest in the use of micro combined heat and power (CHP) technologies in the residential sector. Micro CHP is the simultaneous production of heat and power in a single building based on small energy conversion units. The heat produced is used for space and water heating and possibly for cooling, the electricity is used within the building. In this paper, two typical CHP types for a residential building are analyzed, namely ECOWILL gas engine plant and LIFUEL fuel cell plant. For each facility, two different control strategies are studied: to minimize annual energy cost and to minimize annual CO2 emissions, using an evaluation model constructed in the optimization software package LINGO. The analysis result shows that the fuel cell system serves the assumed residential building better than the gas engine system. With the operation of optimal economic benefits, annual energy cost is reduced by about 26% compared with the conventional system (served by utility grids). On the other hand, considering the optimal environmental benefits, annual CO2 emission is reduced by about 9%.

KEYWORDS Micro CHP, Residential application, Optimization, Operating mode

INTRODUCTION Residence is generally great consumer of energy, both electrical and thermal, accounting for nearly 15% of the total energy use. The power to most residential buildings is supplied from power generation plants over the nationwide electricity grid. The thermal efficiency of the off-site power generation is typically less than 40 percent. Furthermore, the waste heat generated at the utility plant cannot be used effectively. Therefore resources are wasted and excessive amounts of greenhouse gases are emitted. Governmental pressure to increase the efficiency and decrease the emission rates in the residential sector coupled with increasing customer dissatisfaction with the utility grid and demand for independent power generation have led companies to develop on-site energy generation systems [1]. The introduction of micro combined heat and power (CHP) – the simultaneous production of heat and power in a residential building based on small energy conversion units such as reciprocating engines or fuel cells – is of increasing political and public interest. A micro CHP system can supply the electricity needs of the residence while yielding “waste heat” that can be used for space and domestic water heating. The large-scale introduction of decentralized generation units like micro CHP would radically change the electricity system and turn consumers into power producers. The conditions for a widespread implementation of micro CHP and its consequences for the energy market, customers, the environment and the economy require an interdisciplinary investigation. Now, five countries– Germany, Great Britain, the Netherlands, Japan, and the United States of America– are most active in the †

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research and introduction of micro CHP system [2]. In Japan, Osaka Gas has developed the commercialized residential gas engine CHP system in March, 2003. And now a 0.7--1.0 kW class polymer electrolyte fuel cell (PEFC) CHP system is being developed for the Japanese residential market. The Japanese Government, through the Ministry of Economy, Trade and Industry (METI) is actively supporting the commercialization of Residential CHP Systems in Japan. In 2005, METI funded the commercialization process through a US$ 23 million Large Scale Monitoring Program, involving subsidies for approximately 400 CHP systems across the market. As a three-year program, the goal of METI’s Large Scale Monitoring Program is to prepare Japan’s 46-million household market for the full-scale introduction of residential CHP systems [3]. It is believed that micro CHP offers significant benefits to energy suppliers, to household and to society as a whole. With respect to the technological aspect, it is important to underline that for residential end-user, a micro CHP system should be characterized by a low price and easy-to-use operation. Furthermore, attention should also be paid not only to the system itself, but also to the optimum match between the CHP system and the end-use load profile. In this study, for an optimal efficiency of the facilities, a study of various alternative micro CHP systems is undertaken, such as gas engine and fuel cell. In order to utilize the high economical and energy-saving potentials of the micro CHP systems, two different control strategies are studied: to minimize annual energy cost and to minimize annual CO2 emissions. An evaluation model which has been previously constructed in an optimization software package LINGO is employed for analysis [4]. The analysis result shows a solution to be reached that optimizes values, such as economic efficiency and the reduction in the emission of CO2.

METHODOLOGY In order to study the performance of various micro CHP systems under different operating modes, an evaluation model which has been previously constructed in an optimization software package LINGO was employed. Figure 1 is a flow chart illustrating the scheme of the model. It requires the detailed structures and rates of electricity and gas tariff, hourly end-use load data for the customer, government policies for the adoption of micro CHP system, as well as CHP system cost and performance data. According to the selected operating mode, the model, in turn, determines the optimal operating schedule of the micro CHP system and corresponding economic and environmental characteristics. Market Customer

Energy tariff rate Interest rate Subsidy

Electricity, Cooling, Space heating, Hot water

Technology Efficiency, Storage characters

Evaluation According to operating mode

Operating schedule Grid power profile Gas purchase profile

Economic effects Energetic effects Environmental effects

Figure 1. Scheme of the model

SCENARIO DESCRIPTION A detached, single-family residential building has been selected as a typical Japan residence. It is 2 located in Kitakyushu, Japan, having a total floor area of 250 m . A micro CHP system is considered for adoption. The system consists of a CHP plant, a storage tank and a back-up burner. The CHP plant

which is driven by natural gas is used to meet the electrical demand. The role of storage tank is to store thermal energy during periods of low thermal energy demand and to supply thermal energy during high demand. If the heating does not completely satisfy the application needs, a supplementary burner can be used. Similarly, if generated electric power is not enough for applications, the customer may purchase electric power from the utility grid.

Customer load The energy demand of the residence can be divided into electrical demand and thermal demand, which consists of space heating, hot water and cooling load. It should be noted that the cooling load for air conditioning is also an electrical demand. In this study, the daily load demand for space heating, cooling, hot water and electricity in different seasons have been calculated according to Ojima’s hourly, monthly energy unit and yearly energy unit in Kyushu [5]. Various hourly load demands of 8760 hours for a residential building have been assessed. Figure 2 shows the annual cumulative hourly load. 20

Electricity

Cooling load

Heating load

Hot water

2

Load (×10 kW)

15

10

5

0 1

3

5

7

9

11 13 15 Hour in a day

17

19

21

23

Figure 2. Hourly customer load Utility tariffs Utility electricity and gas tariffs are key factors determining the economic benefits of the micro CHP installation. Furthermore, the existence of tariffs that charge more or less depending on the time of day determines the operating schedule of the facility. Electric utility service is provided to Kitakyushu by Kyushu Electric Power Co., Inc [6]. Customers purchasing electricity from the utility are assumed to do so at established tariffs. In this study, a time of use tariff rate for residential customers is applied (See Table 1). For this tariff type, an electricity charge is imposed a variation by season (where summer months are July through September, inclusive) and load period (on-peak, mid-peak, and off-peak). Table 1 Season

Summer

Winter

Electricity tariff Electricity Charge

Load Period

Hour

on-peak

8-11,13-15,18-21

32.014

mid-peak

6-8,11-13,15-18,21-22

20.139

(Yen/kWh)

off-peak

22-6

7.192

on-peak

8-11,13-15,18-21

26.701

mid-peak

6-8,11-13,15-18,21-22

20.139

off-peak

22-6

7.192

Gas service is provided to Kitakyushu by Saibe Gas [7]. Table 2 shows the tariffs for residence in the conventional energy supply system and micro CHP system. Table 2 Types

Natural gas tariff

Gas consumption

Foundational charge

Volume charge price

per month (m3)

(Yen/month)

(Yen/ m3)

0-15

872

207

15-30

872

210

30-100

1501

179

>100

2005

203

----

2856

82

Conventional system

Micro CHP system

MICRO CHP ALTERNATIVES In order to optimize the match between the micro CHP system and the thermal and electric requirements, in this section, different micro CHP alternatives with gas engine and fuel cell are analyzed. For each of the alternatives, two different operating modes: to minimize annual energy cost and to minimize annual CO2 emissions, have been considered. In the first mode, the unit is running intellectualized to minimize annual energy cost, consisting of annualized system investment cost, running cost, fuel cost, and so on. The system operating is taking based only on the reduction in the energy bill. The CHP equipment is assumed to be able to operate anywhere between 0% and 100% of its rated capacity at any time. In this study, the CHP power is not allowed to be exported to the grid. In the second option, the operation of CHP unit is controlled to reach maximum environmental benefits. Annual CO2 emission is composed of emission from combustion of natural gas and emission of grid electricity. Micro CHP with gas engine Gas engine is the most mature of the micro CHP technologies. Such companies as Honda, Aisin Seiki and SenerTec have developed engines with long lifetimes and long service intervals. Emissions are kept low, and acoustic insulation holds noise levels down. Gas engines account for about six of the eight micro CHP products currently available. Five of these generate around 5 kW of electricity, and one generates 1 kW of electricity. The larger-capacity units are typically installed in small businesses and multifamily homes that are able to use all of the heat produced. The internal combustion engine typifies the ECOWILL micro CHP product, producing 1 kW of electricity and just over 3 kW of heat. Detailed technology and market information is illustrated in Table 3. Well over 15,000 of these have been sold to Japanese households and housing developers since the product was launched in 2003, and most Japanese gas utilities are now selling it. Heat is recovered from the engine to supply a home's domestic hot water system and hydronic underfloor heating system. The Honda engine is exceptionally quiet and clean and needs servicing only every 6,000 hours. Table 3

Characters of micro CHP systems [7] Item

Value Gas engine

Fuel cell

870,450

100,000

Efficiency for electricity generation (%)

20

37

Efficiency for heat recovery (%)

65

50

Lifetime (Yr)

10

10

Tank volume(l)

150

200

Capital cost (Yen)

Minimum energy cost In this option, a gas engine facility of 1 kW has been considered. It has been operated to minimize annual energy cost. The hourly electrical and thermal demand and production are shown in Figures 3a and 3b. The CHP system is out of operation from ten at night to seven in the morning. This is because of the relatively low price of the grid electricity during the off-peak time. The benefits from recycled heat can not make up the high generation cost of CHP electricity. From the view point economic, the system prefers to buy the electricity from the grid instead of generating on side. In this case, only 24% of the electricity is generated on site and about 60% of the thermal load is served by the CHP system. This is because of the relatively low electricity efficiency of gas engine plant. 2.0

1.5 Utility grid Thermal load (MW)

Electricity load (MW)

micro CHP 1.0

0.5

0.0

Storage tank

Back-up burner

7

13

1.5 1.0 0.5 0.0

1

3

5

7

9

11

13

15

17

19

21

23

1

3

5

9

11

15

Hour in a day

Hour in a day

(a)

(b)

17

19

21

23

Figure 3. Hourly electricity and thermal balance with gas engine at minimal energy cost operation Minimum CO2 emission This second section, considers the operation of achieving minimum annual CO2 emissions. Figures 4a and 4b show the electrical and thermal balance of demand-production. The CHP system operates at high capacity in the day time and low capacity in the night. This is because of the relatively low thermal load at night. The CHP system is able to provide about 30% the electrical require by the residence, which is more than the case with minimal cost operation. It means sometimes the CHP system is optimal to operate from the viewpoint of environment although it is not economic. Furthermore, from Figure 4b, it can be found that about 75% of the thermal load is served by the storage tank. At the midnight when there is little thermal load, the CHP system still operates at lower capacity, the heat is stored for the following hour use. So, with a storage tank, the peak thermal requirements for space and water heating can be reduced and thermal energy can be better utilized. 2.0

1.5 Utility grid Thermal load (MW)

Electricity load (MW)

micro CHP 1.0

0.5

0.0

Storage tank

Back-up burner

7

13

1.5 1.0 0.5 0.0

1

3

5

7

9

11

13

15

Hour in a day

17

19

21

23

1

3

5

9

11

15

17

19

21

23

Hour in a day

(a) (b) Figure 4. Hourly electricity and thermal balance with gas engine at minimal CO2 emissions operation

Micro CHP with fuel cell The second of proposed solutions is based on fuel cell. Several developers of fuel cell technology are making steady progress on commercializing their products for micro CHP applications, although all need to see further progress on cost, reliability and lifetime. Japan is the current hotbed of activity, with much of the focus on 1kW polymer electrode membrane (PEM) fuel cells, and with companies there already leasing hundreds of units to households. Like the LIFUEL system, as shown in Table 3. These units are supplied with a hot water storage tank and a supplementary burner to meet all of a household's thermal needs. The installations are heavily supported by government funding, and gas utilities aim to start selling systems by the thousands, beginning in 2008. Manufacturers supplying these units are striving to increase the lifetime and reliability of their systems and reduce costs. A fuel cell based micro CHP system represents a particularly promising system for a residence due to its high electrical efficiency, excellent part load performance, small-scale applicability, and quiet operation. Minimum energy cost In this case, the CHP with fuel cell will work in order to minimize annual energy cost. The electrical and thermal balances are shown in Figures. 5a and 5b. Just as the gas engine system (see Figure 4), this CHP system does not work during the off-peak hours. However, during other hours, it operates at rated capacity almost all the time. This is because of the high electricity efficiency of the fuel cell system compared with that of the gas engine system. Furthermore, it can be found that even if it is operated at the rated capacity, it only serves less than half of the total electricity requirements. From figure 5a, it can be seen than about 30% of the thermal load is served by the storage tank. Compared with the electricity generation, it can be deduced that only 79% of recycled heat is used, others is let into the air. 2.0

1.5 Utility grid Thermal load (MW)

Electricity load (MW)

micro CHP 1.0

0.5

0.0

Storage tank

Back-up burner

7

13

1.5 1.0 0.5 0.0

1

3

5

7

9

11

13

15

Hour in a day

17

19

21

23

1

3

5

9

11

15

17

19

21

23

Hour in a day

(a) (b) Figure 5. Hourly electricity and thermal balance with fuel cell at minimal energy cost operation Minimum CO2 emission For this alternative, the fuel cell CHP system is assumed to be operated to minimize annual CO2 emissions. Figures 6a and 6b present the hourly electrical and thermal balance. As shown in Figure 6a, the fuel cell system is operated at rated capacity almost all the time. Only at some off peak time, it operates at part load because of the relative low electricity price. Totally, about 49% of the electricity is generated on site. However, as illustrated in Figure 6b, this type of CHP system offers less thermal load with respect to that offered by gas engines due to the relatively low heat to power ratio of fuel cell plant. Only about 50% of the thermal load is from the storage tank. At the time when thermal load is low, most thermal is served by the storage tank; at other times, the back-up burner takes place. Although the thermal load is very low and even zero at the mid night, the CHP system is stilled operated near rated capacity, the recovered thermal is stored and released during the daytime. However, this inevitably leads to the heat loss. Only about 78% of the recovered heat is valid for the end use.

1.5

2.0

Utility grid Thermal load (MW)

Electricity load (MW)

micro CHP 1.0

0.5

0.0

Storage tank

Back-up burner

7

13

1.5 1.0 0.5 0.0

1

3

5

7

9

11

13

15

17

19

21

23

1

3

Hour in a day

5

9

11

15

17

19

21

23

Hour in a day

(a) (b) Figure 6. Hourly electricity and thermal balance with fuel cell at minimal CO2 emissions operation

ECONOMIC AND ENVIRONMENTAL ASSESSMENT The evaluation of a micro CHP system is a complex and demanding task. The construction and operation of a micro CHP system is a project that must be evaluated in many aspects including economic and environmental. All this factors are necessary and should be taken into account by the decision-maker. In this section, the economic and environmental aspects of each system have been evaluated and the results have been compared. The economic assessment provides information on how the economic resources (investments, fuels, etc.) are used to meet the customer requirements. Figure 7 shows annual energy cost for various systems, and also comparing with a conventional system which satisfies the electric and thermal requirements by utility grid and gas boiler, respectively. It can be found that the introduction of all the micro CHP alternatives leads to cost reduction. The fuel cell system with minimal cost operation has the least annual energy cost, which is about 26% less than the conventional energy system. Furthermore, Fuel cell CHP system has a relatively higher economic efficiency than the gas engine system, although with a larger investment cost. This is because of the higher electricity efficiency which leads to a low fuel consumption for CHP system. On the other hand, as expected, the minimum cost operation is more economic than that of the minimal CO2 emissions operating. However, this is not obvious for both gas engine and fuel cell systems.

Electricity purchase

Investment

Gas for CHP

Gas for burner

6

5

Annual cost (×10 Yen)

8

4

2

0 Conventional system

Gas engine (minimal energy cost)

Fuel cell (minimal energy cost)

Gas engine Fuel cell (minimal CO2 (minimal CO2 emission) emission)

Figure 7. Annual energy cost for various energy systems CO2 reduction is a main consideration for the introduction of micro CHP systems. Figure 8 is annual CO2 emissions of various systems. The adoption of all micro CHP alternatives has an environmental benefit. The fuel cell system with

minimal CO2 emissions operating leads to the largest CO2 reduction, which is about 9% less than that of the conventional system. As to the gas engine system, the CO2 reduction is not obvious for both operations. This is because of the high fuel consumption for the CHP plant and the heat losses of the storage tank.

Annual CO2 emission (t)

12

Grid electricity

Gas for CHP

Gas for burner

8

4

0 Conventional system

Gas engine (minimal energy cost)

Fuel cell Gas engine Fuel cell (minimal (minimal CO2 (minimal CO2 energy cost) emission) emission)

Figure 8. Annual CO2 emissions for various energy systems

CONCLUSIONS In this study, four alternative micro CHP systems have been analyzed. The result can be summarized as follows: (1) With the minimal CO2 emissions operation, the fuel cell system operates at rated capacity almost all the time. (2) With the minimal energy cost operation, the two micro CHP systems operate only in the daytime. The fuel cell supplies more electricity but less heat than the gas engine. (3) From the viewpoint of economic, the fuel cell system with minimal cost operation is the optimal option. (4) From the viewpoint of environment, the fuel cell system with minimal CO2 emissions operation gets the best result.

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