1. Expanding Photovoltaic Penetration with Residential Distributed Generation from Hybrid Solar Photovoltaic Combined Heat and Power Systems
This paper mainly focuses on the potential of taking into action a distributed PV and CHP hybrid system and how it can help in order to increase PV penetration level in the U.S. The installation of such a hybridized section will reduce the energy waste and will also increase the share of Solar PV to be expanded by a factor of 5.
1.1 Technical Limitation to PV penetration in the current grid
At high PV penetration (eg>20%) the cost saved by intermittent load will increase instead of decreasing. The variation in PV power that create this problem are 1)day/night cycle 2)yearly cycle 3)fluctuating cloud condition.
1.2 Electrical and heat requirements of representative U.S. single family
The average annual electricity demand per house in the U.S. is 10654kWh. If we observe the solar flux and electricity demand at various hot and cold region just installing PV system cannot met all the demands. When the PV system is not providing enough power the CHP system will turn on and will maintain a constant load. A CHP+PV system in hotter region can give maximum efficiency if the heat generated by the CHP unit is used for cooling( using absorption chiller) such a system is called CCHP.
1.3 Design of Solar PV and CHP hybrid system
The PV+CHP system consists of 3 technologies:-1)PV array 2)a natural gas engine generator 3)Advanced warm air heating system. Technology Evolution of CHP units:- 0th generation:-1.2kWe CHP and advanced thermal comfort for variable thermal loads.(no electric loads)--Already Available in market. 1st generation:-1.2kWe CHP + 0.6 kWe PV and advanced thermal comfort for variable thermal loads-fixed input for generator heat dumping-load following in backup mode. PV panel converts 20% of the sunlight incident on them rest is wasted. This type of system is 84% efficient and can be compared to 35% efficient conventional power plant for burning same natural gas. 2nd generation:-1.2kWe CHP + 1.2 kWe PV and advanced thermal comfort for variable thermal loads-fixed input for generator heat dumping-load following in backup mode. Future generations:-Adding a Absorption chiller to the system to utilize the CHP produce heat for cooling. Even trying to reduce the wasted energy from the sun this an be done by adding a solar thermal system.
1.4 Sizing of Solar PV and CHP hybrid system
The amount of electricity and heat generated considering CHP system gives full backup to PV system( certain assumptions are made) for 2nd generation PV+CHP system is around 10512kWh per year which meets the electricity demand per household. The PV+CHp system produces 76 Mbtu and 96 Mbtu per year at peak sun hours. In 2nd generation CHP will produce a far more than required amount of heat for certain residential but in 3rd generation this problem will be solved by installing a absorption chiller.
1.4 PV penetration level-Percentage of PV generated electricity
Using various equations and observing average electricity demand at each hour 25% of the total demand can be supplied by PV system in hotter regions an d remaining 75% comes from the CHP system with no storage element included. This will for sure increase the PV penetration in U.S. I order to avoid hourly variation it is better to install a energy storage unit as the response time of CHP unit is not so fast.
2. Institutional scale operational symbiosis of photovoltaic and cogeneration energy systems
The paper mainly focuses on Three design scenarios using only existing technologies for such a hybrid system are considered here:1) single cogeneration + photovoltaic, 2) double cogeneration + photovoltaic, 3) single cogeneration + photovoltaic + storage. Numerical simulations for photovoltaic and cogeneration performance. The paper even shows total amount of natural gas required to provide for the hospitals needs could be lowered from the current status by 55 % for scenario 1 and 62 % for both scenarios 2 and 3, respectively. This significant improvement in natural gas consumption illustrates the potential of hybridizing solar photovoltaic systems and cogeneration systems on a large scale.
2.1 Materials and Methods
In this section the raw data which is important for installing a PV+CHP hybrid system at a location is discussed. In this paper they have considered example of a Hospital. The solar PV component is simulated using Solar flux and temperature data of that location. The CHP unit is simulated using thermal and electric load demand of the hospital in Tehran (Iran).
2.2 Proposed CHP system
CHP unit uses heat exchanger to utilize the waste heat. Thus, we get a overall efficiency of more than 85%. Fuel consumption is very less in CHP system.The total efficiency for a CHP system is given by:η= (Q+E)/Q0 Where Q and E represent utilized thermal and electrical energy, respectively and Q0 shows the heat content of the fuel. Hourly data of thermal and electric load is taken. Then base load of the location is considered. Here the base load for hospital was 300kW. Even the peak hours are identified and here the peak hours are 18-24 hrs where load is 600kW. CHP units consisting of natural gas as source has electrical efficiency of around 30-40%. Out of the wasted heat is almost 90% recovered. Even the paper provide formula to calculate the recovered heat. Along wit this the block diagram of CHP system is also given.
2.3 Solar PV system
The output of solar PV is in the form of Direct current and this cannot be directly connected to grid. An inverter is connected to convert this DC to AC. The PV panels were installed at the roof of hospital in series. Important thing here is the tilt angle of system kept is 10 degrees to ensure self cleaning with rains without penalties. Around 0.5m space was maintained even each panel to ease cleaning. PV technologies works very well where there is good solar irradiance along with it less continuous monthly cloudy days. Such scenario can be seen at borders of Iran.
2.4 Design scenario PV+CHP
It is very important to consider energy efficiency first. Thermal energy consumption can be reduced by installing heat control mechanism which can improve this is done by the CHP system. Electric energy consumption can be reduced by using CFLs. Installing just a PV system can full upto 23% of the electric energy requirement in summer and upto 20% in winter. Design scenario 1: Single CHP + PV
This scenario incorporates a CHP engine capable of matching the 300 kW base load of the hospital.It is expected that the CHP engine will operate at limited capacity during PV electric generation times while operating at full capacity during nighttime. If suppose some extra electrical energy is generated by the PV+CHP system then required, then the excess energy is fed back to the grid. This scenario will suffice for covering the base load of hospital load. Such an arrangement fulfills about 76% of the requirements of the hospital.
Design scenario 2: Double CHP + PV
This scenario incorporates 2 CHP engine each having 300 kW base load of the hospital. This type of arrangement ismore effective and is also complicated. Such an arrangement fulfills about 93% of the electrical energy requirements of the hospital. The remaining energy is fed into the grid this is mainly during the month April and May. The thermal energy requirement of the hospital still are only 56% met.
Design scenario 3: Single CHP + PV + Storage
This scenario incorporates 1 CHP system having base load of 300kW of the hospital. This arrangement is cheaper as compared to scenario 2 and is results are almost similar to result of scenario 2. However, the controls and batteries necessary to smooth out the electrical load are considerable. The only shortcoming of this design is thermal energy requirement of the hospital is not fulfilled completely.
2.5 Results and Discussion
The CHP unit is assumed to work entirely on Natural gas for producing both electrical and thermal energy. Producing energy on site is far more efficient then producing energy at power plant which involves transmission losses of around 26%. The power plant efficiency is less then the CHP unit electrical efficiency which is around 35%. The percentage saving on fuel for all the three above mentioned scenarios can be calculated. For scenario 1 the saving is around 44% and for 2 ,3 it is around 38%.