An Optimized Domestic Chp System With Hybrid Electrical Energy

Transcript

This paper has been published on the journal Energy and Buildings, Vol. 55, December 2012: 361-368 A domestic CHP system with hybrid electrical energy storage X.P. Chen, Y.D. Wang, H.D. Yu, D.W. Wu, Yapeng Li, A.P. Roskilly Newcastle Institute for Research on Sustainability, Newcastle University Abstract CHP (Combined Heat and Power) is widely regarded as one of the most promising technologies to resolve energy-related problems, such as primary energy saving, emission reducing etc. Domestic CHP is the energy system applied to the household sector to supply both electric and heat energy to users. Due to the dissimilar characteristics between household electricity and heat demands, conventional off-grid CHP systems may not satisfy both of the demands simultaneously. This study developed a domestic CHP system in which an engine-based CHP fuelled by biofuels was integrated with a hybrid electric energy storage system and operated under FEL (following electric load) energy management strategy. Experimental tests validate the feasibility of this application and the results show that the system can satisfy the fluctuant energy demands in a domestic dwelling. The overall energy efficiency has been improved by 47.86% compared to conventional CHP. Key words: Domestic CHP, Hybrid electric energy storage, Energy management, Energy efficiency Nomenclature AC CHP DC EES FEL FTL HEES PER RC SC Alternating Current Combined Heat and Power Direct Current Electric Energy Storage Following Electrical Load Following Thermal Load Hybrid Energy Storage System Primary Energy Ratio Resistance and Capacitance Super Capacitors 1 This paper has been published on the journal Energy and Buildings, Vol. 55, December 2012: 361-368 Symbols Storage system charging efficiency at time output with electric power (%) Electric transmission efficiency of the storage system over charge/discharge at time point of (%) with electric power output Electric efficiency of the engine at the time t (s) with electric power output (%) The times of engine starting Heat power recovered from coolant system at time output power with electric (kW) Nominal electric power of the engine (kW) Heat power recovered from exhaust gas at time output power with electric (kW) Maximum Power (kW) Total heat power recovered at time with electric output power (kW) Electric output power of the energy storage system (kW) Discharge power at discharge efficiency (kW) Total heat energy recovered (kWh) Total heat energy recovered from coolant system (kWh) Total heat energy recovered from exhaust gas (kWh) Resistance (Ω) Engine Operational Duration (s) Storage System Discharging Duration (s) Engine operation time during the duration i (s) Peak Hour (s) Open-circuit Voltage (V) Efficiency (%) Average charge efficiency of the storage system over (%) Average electric efficiency of the engine operation (%) Electric efficiency of the storage system (%) Overall efficiency of the system (%) 2 duration This paper has been published on the journal Energy and Buildings, Vol. 55, December 2012: 361-368 1. Introduction The Climate Change Act 2008 launched a trading scheme targeting on 80% reduction in greenhouse gas emissions by 2050 compared to 1990 level in the UK[1]. As the report from the Department for Environment Food and Rural Affairs (DEFRA, UK) pointed out, domestic energy consumption contributed to half share of UK greenhouse gas emissions [2]. Therefore, energy consumption and emission reduction from homes and communities is one of the main sectors to decrease UK emissions[3]. Therefore, development of energy efficient domestic energy supplying systems is one effective way to achieve the target. Distributed domestic CHP is one of the technologies for energy supply on the brink of wide application in Europe, which is regarded as the most appropriate solution. Undoubtedly, domestic CHP should satisfy both electricity consumption and heat demands. However, heat and power consumptions are very different from each other in practice. Domestic heat demands have apparently seasonal difference but the variation over one single day is relatively slow compared to the electricity demands which fluctuate dramatically within each day but have repeatability over the year. Lawson[4] investigated the energy consumption in a three-bedroom house over 24 hours. The result from his study shows that the daily electricity demands fluctuated over a wide range from several-hundred watts for a majority of time to around 7kW for a very short period. In comparison, the heating profile changed rather slowly. Therefore, following electric load (FEL) strategy may be used for the distributed domestic CHP systems preferably due to the complexity of electric demands profile in household dwellings. On the other hand, the electric and heat output of the engine-based CHP system performed differently. Generally, electric efficiency is much more sensitive when load requirement increased. According to the study of engine performance which will be discussed later, electric efficiency increased by a factor of four when engine output varied from 10% to 100%. As a result, engine will be most efficient providing it works on the highest load demands. By contrast, heat recovery efficiency has relatively small variation in which there is only 4% difference as loads rise from 10% up to 100%. Huangfu, Y [5] pointed out the electricity efficiency is the highest influence factor concerning improving primary energy ratio (PER) in CHP systems. He further advised that prime mover should be operated with the electricity output higher than half of the full load. Therefore the engine/generator should work at a relatively high output in order to achieve both higher electric efficiency and lower fuel consumption. However, if an engine/generator is operated with high output, there would be more electricity generated which may be excessive for a household user due to the electricity needed in a household is quite low over the majority of duration in one day, as shown in Figure 1. As a result, the system efficiency for an 3 This paper has been published on the journal Energy and Buildings, Vol. 55, December 2012: 361-368 engine-based CHP system will be very low providing the engine is operated to match the electricity demands. Therefore, it is difficult to find a satisfactory solution to balance the outputs and consumptions of both electricity and heat at high efficiency for a distributed CHP system. A considerable amount of investigations proposed miscellaneous design and solutions for domestic CHP applications [6-10]. Furthermore, researchers, such as Mago, P.J. [11, 12], Piacentino, A[13] and Wang, J.-J[14], did studies theoretically and it seemed that they had found solutions for the problems. So far, however, most of investigations concentrated on the CHP systems themselves statically rather than taking into account for the dynamic consumption of energy in households. Aiming to develop an efficient distributed domestic CHP, this work will design an improved domestic CHP system with electric energy storage (EES) in which dynamic energy consumption in household will be taken into account. The CHP coupled with an EES system comprises of a bio-fuel engine and a hybrid electric energy storage system which is consisting of batteries and super-capacitors. Improved FEL energy management strategy will be used and evaluated for primary energy saving and efficiency improvement in this study. 2. The system of domestic CHP with hybrid electrical energy storage 2.1. Electricity and heat demand profiles of a selected household A typical household in the UK was selected for this study. Figure 1 [4] shows the electricity and heat consumption/demand profile over 24 hours for the selected house. From the figure, it can be seen that the minimum demand of electrical power was around 100W and the maximum demand reached to 6.544 kW. From Figure 1 (a), it can also be seen that the electricity consumption was lower than 1 kW during the majority of 24 hours’ time while the peak demands happened at relatively short periods. For instance, at 1.18pm, the demand was as high as 6.544 kW before it plunged to 400 W 5 minutes later. The peak demands over 2.500 kW appeared 3 times, starting from 5.36am, 1.18pm, and 9.37pm. Electricity consumption for this household over 24 hours is 9.85 kWh in total. 4 This paper has been published on the journal Energy and Buildings, Vol. 55, December 2012: 361-368 Figure 1 (b) demonstrates the heat demand profile for the same house in the same day. Heat energy was used for space heating and hot water. According to the profile in the Figure 1 (b), the maximum heat demand was 1.375 kW while the minimum was 205W. The overall amount of heat requirement was 18.5 kWh for the day. Compared to the electricity profile, heat profile had a slow and moderate variation during the 24 hours where there was no sudden great change from time to time. Fig. 1 (a) Electricity demands Fig. 1 (b) Space heating and hot water demands 5 This paper has been published on the journal Energy and Buildings, Vol. 55, December 2012: 361-368 Gas Walve Biodiesel pipe Exhaust gas heat exchanger AC generator Biodiesel engine set Jacket water pump Exhaust gas Radiator Water Walve Water heating exchanger (tank) Jacket water heat exchanger Space heating Water pump Space heating Hot water loop Tap water Hot water pump Domestic hot water pump Water Tank AC DC Domestic hot water DC Electrical Energy Storage Hybrid System AC Electricity Inverter Converter Fig. 2 Schematic layout of CHP-HEES system 2.2. The design and implementation of the CHP-EES system for the household Based on the heat and power demand profiles of the household, a domestic CHP with hybrid electrical energy storage (CHP-HEES) system was designed and implemented, as shown in Figure 2. The system included an engine/generator with a heat recovery system (CHP) and a hybrid energy storage system (HEES). The system was a distributed/isolated system and allowed different types of energy (electricity and heat) to be generated locally. The engine was fuelled with bio-diesel and was used to satisfy basic electricity demand; the waste heat from the engine cooling system and the exhaust gas was recovered and stored in the form of hot water in a tank; and the hot water was utilised to supply heating and hot water for the house. The HEES consisted of battery bank and super capacitor module. It was connected to the diesel generator. The electric energy generated from the generator but not used at off-peak hours was stored in HEES system and then released along with electricity generated from the generator to satisfy the electricity demands at peak time. 6 This paper has been published on the journal Energy and Buildings, Vol. 55, December 2012: 361-368 2.3. The engine generator and the heat recovery system One engine/generator, which provided a nominal power output = 2.5 kW, was selected as the primary mover of the CHP system; along with engine operation to supply electricity demand, waste heat from both engine cooling system as well as exhaust gas was recovered and stored in a hot water tank to supply heating and hot water. 2.4. The hybrid electric energy storage system The HEES system consisted of batteries and super-capacitors. Batteries are regarded as common energy storage devices which can store and supply large amounts of energy in a relatively small volume. Super-capacitors are used as auxiliary power devices to store electricity. Compared to batteries, they have much lower energy density and may store only limited energy inside. However, their advantages, such as superior power density, long cycle lives and fast charge and discharge duration enable them to become essential components to shave peak and response the fluctuating load rapidly in storage application. Andrew et al [15] discussed in detail the reason why super-capacitors are better than batteries in high power application. Accounting for both batteries and super-capacitors with similar weight, super capacitor module Superfarad 50V/250F with 16kg weight, its energy density is 5.4Wh/kg only but it has 219W/Kg power density. For Varta NiHD battery with weight of 17kg, it has 70Wh/kg energy densities and 46 W/kg power densities only. Furthermore, Equation 1 describes maximum power calculation for batteries and equation 2 represents the practical power value by means of high power pulse. (1) (2) Assuming =95%, the practical discharge power maximum power is equal to 0.19 times nominal . If discharge power is over this limit, the battery performance would get worse. This is due to the unavoidably slow mass transport processes in the 7 This paper has been published on the journal Energy and Buildings, Vol. 55, December 2012: 361-368 battery during charging and discharging. As a result, there may be half of discharge energy being transformed into heat when batteries discharge with maximum power. On the other hand, for super capacitors (SC), the specific energy stored in it is not high because of the limitations in the accessible specific surface area of the electrode in the SC, but the specific power is relatively high because of the short time constant of double layer charging. A small RC time constant (0.1-2s) decides they can response to rapid fluctuation of electric load transiently along with imperceptive heat generated. Therefore, super-capacitors can be used as supplementary power sources to assist the batteries in hybrid applications. Combination of these two different types of electric storage devices can achieve both power and energy advantages which can satisfy both large alternation within short term and great amount of electric energy demands over long period. In this application, the energy storage system consisted of the batteries and super-capacitors as shown in Figure 3. The batteries and super-capacitors were linked together in parallel via DC link box and then DC current released from them was transformed into AC current by the inverter. On the other hand, AC current derived from the engine/generator was converted into DC current to charge the HEES system through the charger. The HEES system consisted of 6 units of AGM 12V/200Ah batteries and a 30V/160F super capacitor module. Circuit break 1 Engine/generator Power meter 1 Batteries current meter 1 DC link box Charger Super-capacitor module Circuit break 2 Load Bank Inverter current meter 2 voltage meter 2 Shunt voltage meter 1 Communication and controller Fig. 3 The layout of the HEES system 8 Power meter 2 This paper has been published on the journal Energy and Buildings, Vol. 55, December 2012: 361-368 2.5. Energy management strategy The most popular energy management strategies used in CHP application are FEL (following electric load) and FTL (following the thermal load) [11-13]. Taking into account electric demand varying much more quickly and more complicated than heating demand in this application, as shown in section 2.1 above, FEL strategy was selected as the energy management strategy. The CHP-EES system here adopts FEL strategy targeting to optimize the overall system energy efficiency where all efficiency calculated by energy gained against fuel consumed. Therefore, the system aimed primarily to satisfy the electricity demand; and secondly to meet the heat demand. Figure 4 illustrates the system operation transmission chart and Table 1 summarizes the variables applied to control procedure in Figure 4. There are three main states and two sub states applied to the system operational procedures. State 1 represents the operational mode in the following way: engine as the main source coordinate with electric storage system to satisfy peak electricity demands. In operation state 2, engine as the main source supply both electric load and HEES charging over a Fig. 4 Operational state transferring diagram 9 This paper has been published on the journal Energy and Buildings, Vol. 55, December 2012: 361-368 number of certain off-peak hours. In operation state 3, electric storage system provides electricity to domestic demand alone in the period of low electricity requirement. Sub-state 2-1 and 2-2 stand for two different charging approaches. Both of them belong to state 2. In sub-state 2-2, EES is charged by default charging curves. Otherwise, it would be charged by the amount calculated by engine power subtracting loads power. The optimization energy management strategies are implanted in control system for governing energy demands analysis as well as energy generation. Table 1 Variables summary Variable name Explanation Note flagEnd State variable (true or false) represents =True Charge end charge state =False In charging Pcharge Default charge power for storage system Default setting charge curve Pgen The maximum value of output power from generator Constant pload Load power demands -- Pstorage Maximum discharge power from storage system Constant 2.6. System operational plan The HEES system supplied the electric power to meet the demand when the load was lower than 4000 W (refer to Figure 1 (a)). The engine/generator ran along with the EES to satisfy peak demands. In order to obtain higher overall efficiency for the CHP-EES system, the engine generator was started and operated 3 times, namely, 6 – 7 am, 13 – 16 pm and 22 – 24 pm. During the running periods, the EES was charged with the extra electricity from the engine/generator, making sure it had enough energy stored and supply electric power as required. 2.7. Waste heat recovery 10 This paper has been published on the journal Energy and Buildings, Vol. 55, December 2012: 361-368 Heat obtained was recovered from the cooling system and the exhaust gas of the engine/generator. Therefore, heat energy recovered was equal to the sum of them over the whole engine operation which was calculated by the series of equations below, Fig. 5 Household electricity demands and supplying profile 11 This paper has been published on the journal Energy and Buildings, Vol. 55, December 2012: 361-368 (3) (4) So, (5) With (6) 2.8. System efficiency Several important efficiency-related indicators can be produced. The calculation is described in the following sections. 2.8.1 Engine electric efficiency The electric efficiency of the engine is the mean efficiency over the engine operation as follows: 2.8.2 The electric efficiency of the storage system The electric efficiency of the storage system is equal to its charge efficiency multiplied by its charge/discharge transferring efficiency. Therefore, electric efficiency can be expressed as equation (8) below: (8) 2.8.3 Overall electric efficiency of the system 12 This paper has been published on the journal Energy and Buildings, Vol. 55, December 2012: 361-368 System overall electric efficiency can be calculated by equation (10), (10) 3. Results and discussions According to the system operational plan and the energy management strategy designed from the electricity and heat demand profiles of the selected house, experimental tests were carried out and the results are shown in Figure 5 and Figure 6. The engine/generator generated all of electric power required by the house including electric energy stored in the HEES system. When the test started, the HEES system was fully charged. Fig. 6 (a) Energy contribution Fig. 6 (b) Operation duration (Hours) 3.1. The efficiency of the engine generator Table 2 shows the test results of the electric and heat recovery efficiency of the engine generator when it runs at different loads. It can be seen that the electric efficiency altered from 7.8% to 28.1% as the loads increased. However, heat recovery efficiency fluctuated within a small range between 34.5% and 41.1% when the load changed widely. 13 This paper has been published on the journal Energy and Buildings, Vol. 55, December 2012: 361-368 Table 2 Efficiency of the engine/generator Engine load (%) Electric efficiency (%) Heat recovery (%) 10 7.8 38.4 25 16.3 34.5 50 24.2 36.4 75 27.3 41.1 100 28.1 41.0 3.2. Dynamic performance of the engine In the tests, the engine started and ran three times when the electric load demands were over 4.000 kW or the remaining energy stored in the EES system was inadequate. Figure 5 (a) showed the electricity consumption profile. The performance of the engine/generator was shown in Figure 5 (b). When the engine/generator was running, it supplied electric power with the assistance from the HEES system during peak time; otherwise, the engine/generator charged the HEES system at off-peak time until the HEES was fully charged. From Figure 5 (b), it can be seen that the engine/generator was always running above half load. Therefore, high efficiency of the CHP-HEES was achieved. 3.3. Dynamic performance of the energy storage system The test results of the HEES system when it was run in the CHP-HEES system are shown in Figure 5 (c) and (d). The negative parts of the batteries in Figure 5 (c) and the super-capacitor power profiles in Figure 5 (d) represent the amount of electric power charged, while the positive ones stand for the amount of electric power discharged. The batteries were charged as the engine/generator started at off-peak hours to make sure there was enough energy stored in the HEES system. From Figure 5 (d), it can be seen that the super-capacitors responded to the load fluctuation speedily. When electric demands varied, it released electricity with maximum capability promptly. For the engine/generator, the transition time during operation represents how quickly the engine responds to load variation, which is normally around 3 to 10 seconds depending on engine types. From the test results, it can be seen that the 14 This paper has been published on the journal Energy and Buildings, Vol. 55, December 2012: 361-368 HEES responded swiftly to the sudden change of loads, which has positive effect to engine operation. Consequently, engine has enough time to alter its output properly. 3.4. Heat recovered The total heat energy recovered from the engine during the 24 hours’ test is 21.07kWh, while the heat consumption for the day is 18.5 kWh [4]. The heat recovered is 14% more that the demand. Furthermore, due to the engine being operated at high loads, heat at higher temperature can be obtained for use. Therefore, the practical heat recovered in the test can fully satisfy the requirement for the house in the 24 hours. The heat recovered is 40.1% of the fuel input. 3.5. Electric efficiency and overall efficiency of the system In the CHP-HEES system, an engine/generator rated 2.5 kW is used and coupled with the HEES system. All of the test results are listed in Table 3. Discharge efficiency of the storage system, engine efficiency and overall electric efficiency are 24.01%, 26.45% and 23.59% respectively, as seen in Table 3. For comparison, a CHP system without EES is calculated. The CHP is assumed to have a maximum power output 6.5 kW, to meet the maximum electricity demand; and it has similar electric efficiencies at different loads as the engine/generator in the CHP-HEES system. The simulated results of this CHP system are also listed in Table 3. From the results, it can be seen that the average efficiency of the CHP system without EES is only 4.95%. This is because the engine always runs at very low loads (< 1.000 kW). The heat may be recovered from the CHP is 75.01 kW. It is 4 times that of the overall amount of heat requirement (18.5 kWh) for the day. That means there is 56.51 kW of heat wasted. Compared to the CHP without EES system with 4.95% electric efficiency, the CHP-EES system has a much higher electric efficiency, that is 23.59% overall, which is 4 times more than that of CHP without EES. Furthermore, the overall CHP system efficiency increased from 43.06% to 63.67% where the improvement of the overall system efficiency is 47.86%, as shown in Table 3. 15 This paper has been published on the journal Energy and Buildings, Vol. 55, December 2012: 361-368 Table 3 Performance comparison between two CHP systems CHP without EES CHP-EES Improvement (%) Engine nominal Power (kW) 6.500 2.500 -- 0 – 6.500 0 – 2.507 -- 24.00 6.89 -- (hours) -- 6.48 -- Storage system discharge duration -- 17.52 -- Storage system charge efficiency (%) -- 26.10 -- Storage system discharge efficiency (%) -- 24.01 -- Average engine generator efficiency 4.95 26.45 434.34 4.95 23.59 376.47 75.01 20.07 -- 38.11 40.08 5.17 43.06 63.67 47.86 Operation range (W) Engine operation duration (hours) Storage system charge duration (%) Overall electric efficiency (%) Heat recovered (kWh) Average heat recovery (%) Overall CHP system efficiency (%) From the results, it is found that the engine generator supplied 36% of the electric energy in 24 hours; the HEES system contributed 64% electricity needed. Figure 6 (a) shows the percentage of electricity supplied from the engine/generator and the HEES system. Figure 6 (b) shows the allocation of the operational duration of the engine/generator, the HEES and both of them. The HEES system supplied power for 17.11hours at low load demand period; while the engine/generator worked for 6.48 hours and supplied power and recharged the HEES system; only within 0.41 peak hour, the engine was running together with the HEES system to supply the peak demands. 3.6. System economic analysis The capital of CHP-EES system costs around £6800. According to daily electricity demands, the engine will be operated for 6.89 hours with total fuel consumption of 4.88Kg to satisfy energy requirement, including both 9.85 kWh electricity and 20.07 16 This paper has been published on the journal Energy and Buildings, Vol. 55, December 2012: 361-368 kWh heating energy in this case study. Therefore, fuel costs will be around £2.44 for sunflower oil or rapeseed oil (50p per kg) as estimated. In the UK, there are 1.5 million homes without access to the natural gas grid where the most popular option for home heating is heating oil fired boilers. According to relevant investigations, the overall fuel efficiency by using heating oil is only 48% on average[16] and the price of heating oil is around 53 to 65p (VAT exclude) per kg[17]. Therefore, the cost for satisfaction of heating demands 20.07 kWh will cost £2.22 to £2.72 where they have to pay extra to purchase electricity from power grid or alternative way. Another popular option for energy supplying system in the UK is solar energy system, the capital of a domestic solar energy system with size of 4-10kW capacity in the UK costs between £10,500 and £46,000. Additionally, the maintenance cost is around £3000 over 20 years’ operation[18]. Furthermore, the solar energy system strongly relies on seasons and needs to be connected with power grid. And it is not suitable for stand-alone energy supplying application without the power grid connection. Compare to the solar energy system, CHP-EES has obvious advantages including:    Much cheaper capital cost Free of season-dependence Free of the power grid connection. 4. Conclusions The detailed study of a domestic CHP-HEES system shows that:  A CHP system integrated with a hybrid electric energy storage (HEES) system, a smaller engine/generator can be used to satisfy the same energy demands of the conventional off-grid/distributed CHP system. The smaller engine/generator is able to work at much higher electric efficiency due to the effect of peak shaving and valley filling from the HEES unit.  The integrated CHP-HEES system can satisfy both electric and heat demands required by the house at high efficiency.  Overall energy efficiency increases to 63.10% with improvement by 46.89%, compared to the conventional CHP only system.  CHP-EES system has much cheaper capital cost comparing to solar energy system and more suitable to install in domestic applications. 17 This paper has been published on the journal Energy and Buildings, Vol. 55, December 2012: 361-368 Obviously, biofuel CHP-EES is an ideal option for stand-alone energy supplying system without grid connection and/or gas grid in rural area. CHP-EES features with benefits including rational capital and operational costs. 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