An Innovative Heating System: Performance and Economic Viability
ASAD A. SALEM*
FennCollege of Engineering,
ClevelandStateUniversity,
Cleveland, OH - 44115
Taysir Nayfeh,
FennCollege of Engineering,
ClevelandStateUniversity,
Cleveland, OH - 44115
Shadi Al-Shakaa,
FennCollege of Engineering,
ClevelandStateUniversity,
Cleveland, OH - 44115
Abstract:- This work describes an innovative space heating and cooling system. The proposed system consists of a fuel cell and a heat pump integrated together to form a single heating and cooling system (FCHP). This integration yields a combined heat and power system that generates electrical power on site and recovers the waste heat from the fuel cell. The performance and the economics of this new integrated system were evaluated and compared with conventional space heating systems. The performance of this system was 40-60% higher than conventional systems and its operational cost was lower by 60-70%. This work suggests that in the near future this system could replace conventional heating systems in most climates, especially where heat pumps are already feasible.
Key-words:- Fuel cell, Heat pump, Cogeneration system
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1 Introduction
Fuel cells are electrochemical devices that convert chemical energy (hydrogen and oxygen) to electrical energy and heat. They are clean, quiet, and efficient. They have excellent reliability and long operating lives. In addition to pure hydrogen, natural gas, methanol and other petroleum products can be used as fuel with the use of fuel reformers. Fuel cells in general have an electrical efficiency of about 40%; the other 60% is converted into waste heat [1].
A heat pump is an electric system with the capability of providing both heating and cooling. In the most basic sense, it is a little more than a conventional air-conditioning system that is equipped with the necessary components to cause it to reverse its running cycle. While the heat pump is operating normally, it absorbs heat from the air and releases it inside the building.
The heat delivered by a heat pump is theoretically the sum of the heat extracted from the heat source and the energy needed to drive the cycle. The steady-state performance of an electric compression heat pump at a given set of temperature conditions is referred to as the coefficient of performance (COP). It is defined as the ratio of heat delivered by the heat pump and the electric energy supplied to the compressor. The performance of heat pumps is affected by a large number of factors. For heat pumps in buildings these factors include: the climate, the temperatures of the heat source and heat distribution system, the sizing of the heat pump in relation to the heat demand, the operating characteristics of the heat pump and the heat pump control system. Air-source heat pumps generally have COPs ranging from 1.0 to 4.0 [2].
Current heating systems have an overall efficiency that range from 60% up to 85%. These systems rely on the grid electricity and/or oil or gas. As for gas or oil-fired furnaces their efficiency depends on the age of the system, condition, and performance. For home heating systems operating at 85% combustion efficiency, the DOE reports that the overall efficiency of these systems range between 56% to 76%. In the past, the cost of energy was considerably lower than today and the ratio of energy expenditure was low, so the efficiency of home heating systems was not a major issue to the consumer. But the increase in the price of crude oil and natural gas made the efficiency of the heating systems a major concern.
2 The Proposed System
To cultivate the advantages of fuel cells and heat pumps, we proposed the system shown in Figure (1). In this system the fuel cell and heat pump (FCHP) are combined together in one package. The fuel cell is the source that provides the electricity needed to run the heat pump. At the same time, the heat rejected (wasted) from the fuel cell will be recovered by the heat pump. This recovered heat is added to the heat transferred by the same heat pump from a low-temperature medium. Then the total heat is used to heat the intended space. As shown in Figure (1), the available commercially heat pump has two evaporators (I & II) connected in parallel. The existence of two evaporators eliminates any additional cost for a heat recovery system from the fuel cell. Evaporator (I) will be used to recover the waste heat from the fuel cell while the other recovers heat from the low–temperature ambient air.
For this application, the proton exchange membrane fuel cell (PEM) was selected; but other fuel cells can be used. PEM fuel cells have high power density and can vary their output quickly to meet shifts in power demand. They are relatively small in size, have low material cost than other fuel cells, and have high performance and high-volume manufacturability, which make them ideal for this application. The proton exchange membrane (PEM) fuel cell operates at a temperature in the range of 1200F to 1900F and at pressures of 1 to 3 atm [3].
The goal of this proposed system (FCHP) is to replace the conventional heating systems used in residential/commercial buildings. This research was conducted on the premise that due to the high conversion efficiency of the fuel cell, the FCHP system will be more energy efficient than the separate generation of electricity and thermal energy.
The main sources of the rejected heat in fuel cells are: fuel reformer, fuel burner, and cell stacks. In the FCHP system the rejected heat from the fuel cell is recovered by evaporator (I) and then mixed with the heat absorbed from evaporator (II) of the heat pump and directed to the compressor of the heat pump. Throughout the heating season PEM fuel cells reject heat at a constant and higher temperature than the ambient. For instance the temperature of water generated from the fuel cell is about 1500 F.
3 Performance Analysis
The performance of the FCHP system can be influenced by four primary factors: the heat pump machine, the fuel cell, the coupling between the heat pump and the fuel cell, and the ambient conditions.
The fuel cell is the largest energy consumer in the system. Natural gas is the fuel selected for the PEM fuel cell, therefore a fuel reformer is needed to extract high quality gaseous hydrogen from the fuel. In an overall energy balance the rejected thermal energy from the fuel cell is recovered by the heat pump. The rated net electrical power and thermal energy generated by the fuel cell used in this application are 10000 watts and 38380 Btu/hr at 40% efficiency. This size was selected so the FCHP provides the required heating load when the COP of the heat pump drops to 1.0 at ambient conditions.
The heat pump is also a large energy consumer in the system. It gets its energy from the fuel cell. In addition to thermal energy transferred from the fuel cell, it also transfers thermal energy from the ambient, and transfers the sum to the heated space. Its performance is a function of the rated efficiency of the machine and the temperatures of the ambient and rejected heat from the fuel cell. The most important strategy in assembling an efficient system is to start with an efficient heat pump. The proposed heat pump has a COP of 1.0 at 15 o F and 3.75 at 120 o F [4].
Evaporator (I) is designed and coupled with the fuel cell in such away to recover the heat rejected from various components of the fuel cell. An experimental work showed that evaporator (I) can recover 90-95 % of the wasted heat. Since the fuel cell and the heat pump are packaged in one unit, some redundant components could be eliminated.
The ambient conditions affect greatly the performance of the heat pump to an extent that it becomes highly undesirable when the ambient temperature drops below 20o F. The major problem with air-source heat pump is frosting, which occurs in humid conditions when the temperature falls below 34o F. The frost build-up on the evaporator coils is undesirable since it disrupts heat transfer. The coils can be defrosted, however, by reversing the heat pump cycle. This results in a reduction in the performance of the system. In the FCHP system these problems do not occur in evaporator (I). If such conditions occur in evaporator (II), super-heated refrigerant coming from evaporator (I) can be diverted to evaporator (II) to be defrosted. Our experimental work showed this is feasible without any reduction in the performance of the system.
4 Case Study
In order to assess the performance of the proposed system (FCHP) and compare it to conventional heating systems, a case study approach was employed. The conventional systems used for comparison were an air-source heat pump (HP) and a natural gas furnace (GF) with air conditioner. The case study was done for Cleveland-Ohio. Cleveland was selected because: (a) it is in the northern part of the United States, in which the heat pump is least competitive, (b) the heating load is large, and (c) energy prices are average compared with other parts of US. The study was limited to a residential building with a 75000 Btu/hr heating load.
The operating characteristics of the heating equipment as a function of the outdoor temperature are required 5. The efficiency of a gas furnace is relatively independent of outdoor temperature; however the COP of a heat pump is greatly dependent on outdoor conditions, and this must be taken into account. Another factor that should be considered for all equipment is the effect of operating at a partial load. The energy-estimating method employed to include the characteristics of the outdoor and partial load conditions was the BIN method.
The BIN method is based on the concept that all the hours during a month, season, or year within which a certain temperature (BIN) occurs can be grouped together and an energy calculation made for those hours with the equipment operating under those conditions. A part-load factor (PLF) is defined as
PLF = TEPL/ AEPL
The theoretical energy required part load (TEPL) is based on the steady-state operating efficiency or COP in the case of a heat pump. The actual energy required part load (AEPL) takes into account the loss in efficiency or COP due to shutdown. The PLF may also be expressed as
PLF = 1-D [1- (heating load/ unit capacity)]
Where D is the degradation coefficient, taken as 0.25 as a default value. PLF may also be expressed as [5]
PLF= theoretical run time (tt) / actual run time (ta)
The total energy use for the gas furnace base on the annual BIN data for Cleveland, Ohio was estimated. However, for the heat pump (HP) and combined system (FCHP) a weighted COP was estimated. The weighted COPbin was estimated by the following formula:
COPbin ={(COPbinta)} / ta
Where COPbin is the COP at particular bin temperature, and ta is the actual run time at particular BIN temperature.
Figure (2) and (3) shows the actual running time for the three heating systems. The FCHP system can meet the AEPL by using solely evaporator (I) when the heat load is smaller than 90% of total wasted heat from the fuel cell. Since evaporator (I) is exposed to a constant temperature (140o F) heat source the COPbin for FCHP was considered to be 3.5. When the AEPL is higher than 90% of wasted heat, the FCHP has to use both evaporators, in which case the COPbin the FCHP is actually an average quantity for the two evaporators. The COP of HP and the COPbin FCHP for the total range of BIN temperatures are shown in Figure (3). The average COPbin for the HP is 1.76 and for the FCHP is 2.82.
5 Economic Analysis
Zero emission power generation at high efficiency is actually the main force pushing fuel cell technology toward commercialization. The main factors that affect the commercialization potential of fuel cells are performance, reliability, durability, cost, fuel availability and cost, and public acceptance.
Energy partners has developed PEM fuel cells and brought them to the verge of commercialization. The fuel cell stack consists of 40 cells and is capable of generating 10kW electrical power output at 40% efficiency at1500 F using natural gas or liquid fuel and air as the reactants. The fuel cell stacks are currently being produced on a one-by-one basis as prototypes at a production cost of $4000/kW. Because of their simplicity, fuel cells are ideal for mass production, which will significantly reduce their cost. PEM fuel cell is the most promising and rapidly advancing fuel cell technology for transportation and stationary applications [3].
In order to become competitive with existing technologies, the cost of fuel cells would have to be reduced below $1000/kW. However, this does not take into consideration the fuel cells advantages such as the capability to operate on non-fossil and renewable fuel. The Strategic Center for Natural Gas (NETL) of DOE projects that this cost will be achieved by 2006, and it will drop to $400/kW by 2010 [6].
Barbir and Gomez [7] presented a study of PEM fuel cell economics based on a theoretical fuel cell operating on pure hydrogen. In this study, and with the assumptions of the stack cost to be $2000/kW and a hydrogen cost of $20/GJ, the electricity cost was estimated to be $0.24/kW to $0.6/kW, depending on the operating conditions of a 10 kW fuel cell stack. However, with the expected improvements in the performance and the reductions of the cost of the cell and the fuel, electricity cost is expected to be as low as $0.08/kW to $0.12/kW.
The inputs used to determine the costs and savings for the system are: the building electric load, the building thermal load, the efficiency/COP, other information of the three systems, and the energy cost. These inputs are used to estimate the annual energy consumptions for the three systems, the annual cost and savings, the pay back period, and the present worth value for each system.
The payback period is the amount of time required to repay the extra cost of equipment through the savings realized from using a new system as compared to conventional system. The payback period is calculated by the following formula [2].
Pb = (A-B)/(C-D)
Where A the installed cost of new system, B is the installed cost of conventional system, C is the operation cost of the conventional system per year and D is the operation cost of new system per year.
The installation cost for the HP and the GF are: $4000, $5500 respectively. While, in the case of the FCHP are $15000 at a rate of $1000/kW and $9000 at a rate of $400/kW. The calculations are based on the current energy cost including delivery cost in Cleveland of $ 0.12/ kW for electricity and $10/Mbtu for natural gas. The effective inflation annual rate is 5% and the expected lifetime for each system is 15 years.
The payback time and present worth value are shown in Figure (4) and Figure (5) respectively.Using current energy prices, the annual operational cost for HP, GF and FCHP are: $3265, $2164 and $724 respectively. However, the operational cost for FCHP could be further reduced as the follows: The actual running time for FCHP in this proposed system is a about 2178 hours per year, which leaves 6582 hours per year that the fuel cell is not in operation. Then the fuel cell could be utilized to provide electrical power, at lower cost, for other domestic usage. However, this utilization may reduce the lifetime of the fuel cell.
This economical study shows that at the current cost of fuel cells, the best choice considering the capital investment and the operation cost is to use the gas furnace and an air-conditioning unit. However, as the cost of fuel cells decreases the FCHP system becomes more appealing, with a lower present worth value of $ 22,514 at a cost of $1000/kW for the fuel cell and $16,514 at a cost of $400/kW.
6 Conclusion
In this work, the performance and the operation cost for a heat pump, a gas furnace and the FCHP system were compared. It proved the FCHP system is the most efficient one. The operation cost of FCHP system is the lowest of the three systems.
This study showed that at the current time, the proposed FCHP system is not cost effective. On the other hand, when the cost of fuel cells drops below $1000/kW, the proposed FCHP system may be the most feasible option of the three systems.
In addition, the proposed system may help in opening new horizons and new markets for heat pumps, making it more cost effective in colder climates. In addition, it will expand and create new applications and markets for fuel cells. The next step for this work should focus on building a prototype system to simulate the process. This will help in adding adjustments to FCHP system.
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References:
[1] Thorstensen, Bernt, “A parametric study of fuel cell system efficiency under full and part load operation”, Journal of Power Sources, 92, 2001, 9-16.
[2] Langley, Billy, Heat Pump Technology, Third Edition, 2002