Combining a ‘trigeneration’ (heat, power and cooling) system with a ground source heat pump and other building services plant can deliver some very impressive energy and carbon-saving results. Birol Kilkis describes such an installation at Ankara University in Turkey.
A ‘nested’ trigeneration system coupled to a ground source heat pump delivers heat, cooling, and power to a new research facility built at a university campus in Ankara, Turkey. This building is owned by the Turkish Red Crescent and is mainly used for vaccine development and cold storage. This new building is attached to the Sustainable Building Research Facility at the Faculty of Architecture, in which very recently a CHP unit has been installed. The two buildings are attached by an archival library building. These three buildings have different functions and different power and energy demands. This makes it essential to nest the concept of trigeneration with conventional boilers and chillers in order to optimally meet the different demand profiles. Solar PV and flat plate collectors systems, a small-scale wind turbine will complement the system at various stages.
The objective in this application is to minimize the direct and avoidable carbon emissions. The CHP unit was designed to accommodate a future bottoming cycle, thus the second phase of the project will form another nested loop within itself, namely a ‘polygeneration’ system. The trigeneration system delivers heat and power to all three buildings and a ground-source heat pump driven by the local power delivers comfort heat and cooling that is topped-up by a small chiller for cold storage of the vaccines.
The International Energy Agency has projected that cogeneration systems may reduce carbon dioxide emissions by 0.3 Gt/year in 2050. This contribution may increase to 1.4 Gt/year, if decentralized CHP systems, especially in polygeneration format, can widely penetrate the building sector. When rational exergy management efficiency benefits are included, the actual carbon dioxide contribution may increase to 3 Gt/year in 2055.
Directive 2004/8/EC is a response to encourage a shift towards efficient energy production plants, including CHP systems. The power-to-heat ratio, C is an important parameter concerning efficiency, emissions, and demand-following algorithms. This directive defines the primary energy savings (PES) ratio in term of heat and power generation efficiencies if the CHP plant is compared to separate heat and power generation default efficiency values.
The savings potential is limited to about 37.5% in the practical range of C. This potential is even lower if default range of C, which is between 0.6 and 0.95 is considered. The EU directive depicts a minimum PES value of 10% to rate a CHP system to be high efficiency system.
PES value increases with C, and this paves way to polygeneration systems. The higher the C value, the higher is the rational exergy management efficiency too, which is a metric for the balance between the supply and demand of useful work potential in a given mixture of decentralized energy and power systems in the built environment. Cogeneration systems, due to their increased overall energy and exergy (useful work potential) efficiencies regarding to the fuel(s) used, are expected to reduce the effects of carbon emissions substantially.
The positive impact against global warming by introducing cogeneration and trigeneration systems will be relatively more profound, if optimum designs and strategies are developed and implemented, especially for the building sector.
SUSTAINABLE BUILDING RESEARCH LABORATORY BUILDING
This building, that potentially encompasses all aspects of green and high performance sustainable buildings, aka MATPUM building, was conceptualized, implemented, and commissioned in 2006 by Professor Haluk Pamir, the Dean of the Faculty of Architecture at Middle East Technical University (METU). Several academicians and industry partners were involved. Since then several environment-aware projects have been completed.
|HEGEL project 120 kWe CHP unit before final installation on-site|
This building is a low-exergy building that not only demands less energy but also demands less quality of energy. For example, solar heat that is obtained from flat plate collectors is relatively low-exergy source and may only be balanced on the demand side by a low-exergy building. The MATPUM building encompasses several passive and active solar systems.
HEGEL POLYGENERATION PROJECT
MATPUM research building was very recently furnished by a cogeneration unit, at the end of the year 2009. The installation phase is shown in the photograph on this page.
This system has 120 kWe (power) and 175 kWh (heat) generating capacity at full load. The second phase of this installation will factor in a bottoming cycle with a reciprocating steam engine, 27 kWe secondary power generator, and capillary tube type steam generator from the exhaust heat. In this final set up, the power to heat ratio will approach one. This project is the HEGEL project that was coordinated by the Research Center of Fiat in Torino, Italy.
STATE-OF THE ART LIBRARY OF ARCHIVAL COLLECTION
The MATPUM building has a single-storey library of archival nature that stretches between the MATPUM building and the new vaccine development and research lab owned by the Red Crescent of Turkey (TUKAM Building) with cold storage facilities for vaccines. This library is heated, cooled, and humidity-controlled by radiant floor heating, radiant ceiling cooling, and on-demand radiant electric spot heating. The idea of this complex thinking is the fact that books of archival nature and manuscripts are very sensitive to humidity control and, on top of that, the air temperature.
The lower the air temperature is, the longer the half-life of the books and manuscripts. Because the library is open to circulation, human comfort is also an important parameter of design.
In order to optimize the operation without compromising the books and manuscripts’ quality and lifespan a radiant-convective split of heat transfer was implemented. In this design the air temperature is continuously kept at 16°C in winter and 20°C in summer. Relative humidity is maintained at 50±0.1%. Human comfort is satisfied by radiant panels, which are not seen by the books and manuscripts in specially designed shelves.
The concept is shown in Figure 1. In this section, the valuable books and manuscripts are shelved in an open-to-circulation manner on inclined shelves which mask the thermal radiation from the floor heating system that is grouped in three groups such that books are not exposed to thermal radiation. These groups are separated by thermal insulation such that cold floor strips are generated on the floor. The innermost strip is for patrons who select books and browse them at the wall mounted shelves. These patrons pick up the radiant heat from the innermost floor heated strip but the books on the shelves do not.
|Figure 1. Hybrid library indoor climate control system for collection preservation and comfort.|
Human comfort is practically the average of the dry-bulb air temperature and the radiant surface temperature. The radiant strip counterbalances the cold air temperature (16°C in winter) to maintain human comfort. The comfort band concealed under patrons’ chairs and tables located in the mid section complements human comfort while patrons sit and read the library material. Tables and chairs mask the thermal radiation from the collections on the shelves.
On-demand infrared electric heaters on the innermost corner of the ceiling corner are motion controlled such that they further complement human comfort if necessary without affecting the books on shelves and on the table. Reading material on the table is shielded from thermal radiation by low-profile table shelves and LED reading lighting apparatus. The outer band reduces the window condensation risk at cold weather.
In fact the west-looking windows are triple-glazed, very low U-value panes with two additional layers. One layer is the transparent PV layer, which contributes to green energy scheme of the building complex. The secondary layer is an automatically controlled shading mechanism that is operated by part of the PV-generated dc power. Solar shades that operate to control the solar gains, especially in summer, complement the window shading mechanism. Again the outer floor band thermal radiation is masked by the shelf inclination.
In summer, radiant cooling component is not a problem and therefore the radiant cooling component is below the ceiling and is also observed by the books on the shelves and on the reading tables. The ceiling panel system is composed of active chilled beams and radiant cooling panels. A direct outdoor system is not possible due to strict humidity limits for indoor preservation function of the manuscripts. Therefore a hybrid ventilation and humidity control system is employed. Concealed ducts on the floor and below the ceiling at the shelf side of the library accomplish this function.
RED CRESCENT R&D BUILDING
This super-insulated low-exergy building requires 65% less energy to maintain. On top of this, the energy and power comes from the nested trigeneration and solar/ground/wind energy system. Windows are designed to generate 3 kW DC power at peak solar insolation. The useful floor are of this building is 370 m2. A roof-mounted 400 W capacity wind turbine will complement the power supply.
On the south side of the building a trombe wall system is employed. Solar lighting will be provided to the inner zones of the building, supplied by solar tunnels of special design that incorporate transparent PV panels and lighting control on their cupolas. This new solar lighting system comprises the third hybridization nest of the overall trigeneration system. The system is shown in Figure 2. It serves for both sun lighting, power generation, and heating on demand (in winter).
|Figure 2. Solar light cogeneration system with active shading|
NESTS OF TRIGENERATION
The building complex encompasses three different functions:
- office and research activities
- (MATPUM Building)
- library functions (Library)
- medical R&D with cold storage (TUKAM Building).
All these different building functions level off the power, heat, and cooling loads appreciably, and therefore the equipment was selected on a baseload and power algorithm. The core of the system is the HEGEL CHP unit, which generates 125 kWe and 175 kWh. The previous natural-gas fired condensing boiler will stay in the mechanical room for peaking and back-up functions.
In the second phase of the application a Rankine cycle steam power generation system will add on more electric power capacity by 27 kWe, but reduces the heat supply down to about 140 kWh capacity. By this bottoming cycle solution the power to heat ratio approaches to one and improves the rational exergy management efficiency – see Table 1. This reduction in heat supply is off-set by a ground-source heat pump (item 5 in Figure 3) with a high COP value of 4.5 on average for heating and 3.5 for cooling. The heating capacity of the heat pump is about 25 kW heat and is driven by the local power generated by the CHP unit.
Figure 3. Heat pump and CHP integrated nested trigeneration system with ice storage.
The MATPUM building is not actively cooled for comfort purposes thanks to its passive architectural design. Therefore in summer all the heat supply goes to the TUKAM building for absorption cycle cooling (item 2) both for comfort and cold storage purposes. Ground-source heat pump and absorption cycle cold outputs are stored in a chilled water/ice storage tank (item 3) in order to further level-off the peak cold loads. Because cold storage requires sub-zero temperatures, a conventional small chiller unit (item 4), again driven by the local power is added to the system.
Item 1 in Figure 3 is the flat-plate heat exchanger, which separates the closed-loop hydronic circuit of the TUKAM building from the rest. Part of the cooling is served to keep the library cool at the required indoor dry-bulb temperature. The air cooling system is driven by the ground-source heat pump. The hydronic panel floor heating and ceiling cooling are tailed into the return circuits. Electric infrared heaters are also served by the local power.
Wind and solar PV systems complement the overall hybrid system that has three nested loops on the supply side.
On the demand side, comfort, storage and preservation functions comprise air supply and humidity control system driven by the ground-source heat pump, floor heating and ceiling cooling panels and active chilled beams, on-demand infrared heaters, fan coils (heating only) in the MATPUM building and door-hot air curtains (hydronic). Artificial lighting need is minimized by proper architectural design and active solar shades and solar tunnels. Artificial lighting needs are satisfied by high efficiency bulbs and LED lamps.
This application has demonstrated that different building types and functions may be satisfied for their different loads by a single-source decentralized trigeneration system that is to be designed for base loads. Remaining peak loads may be addressed by conventional system like a condensing boiler and a chiller besides a properly sized thermal energy storage system.
The variety of green mechanical system bundles described in this article enable the designer to freely optimize the system performance and cost. In this application the primary objective was to minimize the carbon emissions, yet the simple payback period is estimated not to be more than 3.5 years thanks to overall optimization.
The progressive benefits of nesting various nests of trigeneration and CHP functions are shown in Table 1. This table reveals that while the primary energy savings are improved appreciably with this progress of nesting, carbon emissions also reduce by more than three-fold compared to a simple CHP unit used alone.
When such a nesting concept of trigeneration is applied to a high-performance, low-exergy building the net system approaches to a true net zero energy building, particularly when the primary fuel input is replaced by alternative fuels like biogas.
Birol Kilkis is with Baskent University, Ankara, Turkey.