The Hybrid Geoexchange Primer. August 2015

Transcription

1 The Hybrid Geoexchange Primer August 2015

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3 The Hybrid Geoexchange Primer How to reduce costs, improve efficiency, and design more robust ground source hea ng and cooling systems GeoEx Analy cs, Sterling, Virginia cs.com

4 Copyright 2015 by GeoEx Analy cs DISCLAIMER GeoEX Analy cs offers the concepts, ideas, and data contained herein for informa onal purposes only. The appearance of any technical data or editorial material in this publica on does not cons tute endorsement, warranty, or guarantee by GeoEx Analy cs of any product, service, process, procedure, design, or the like. GeoEx Analy cs does not warrant that the informa on in this publica on is free of errors. The en re risk of any informa on in this publica on is assumed by the user. cs.com

5 Contents Geothermal as Geoexchange... 1 The Problem of Balance... 2 Solu ons to Imbalance... 3 The GXAc ve Hybrid Solu on... 4 GXAc ve Bore Op miza on... 6 Taking Geoexchange to the Next Level cs.com

6 Geothermal as Geoexchange Geothermal hea ng and cooling uses a water source heat pump to provide heat in the winter and cooling in the summer. Figure 1 shows a typical applica on where water is circulated through a ver cal closed loop heat exchanger to extract heat in winter and reject heat in summer. The natural ground temperature is close to the mean annual air temperature, providing rela vely cool ground temperatures in the summer for efficient heat rejec on and warm ground in the winter for efficient heat extrac on. But this simple picture hides an inconvenient truth: once a ground source heat pump starts, the ground is disturbed, and the constant temperature of the ground is no longer constant. Below the Figure 1. Geothermal cooling and heating via a vertical closed loop heat exchanger. ground surface, there is no wind to blow heat away. Although the ground has a tremendous capacity to store and hold heat, heat moves slowly through the ground. As a result, the op mum use of ver cal closed loops is for seasonal thermal storage rather than long term heat rejec on (or extrac on). Figure 2 shows hourly hea ng and cooling loads and ground loop temperatures where seasonal heat transfers to and from the ground are balanced. During the hea ng season, heat withdrawal causes temperatures to drop in the water and in the ground around the bores. During cooling, heat is returned, temperatures rise, and at the end of the year the ground is thermally restored. The cycle can repeat indefinitely. The peak cooling load of about 100 in this example corresponds to 150 feet of boring per ton peak cooling. The maximum and minimum entering water temperatures (EWTs) are 92 and 30 F. The load weighted cooling and hea ng EWTs, however, are much more moderate, at 76 and 44 F, resul ng in good average annual efficiencies. If the boring footage is increased, the daily and seasonal Figure 2. Hourly Loop Loads (Top) and Temperatures (Bottom). Heat transfers to and from the ground are balanced over the year. Cooler water extracts heat from the ground in the winter and warmer water rejects heat back into the ground in summer. Minimum and maximum entering water temperatures are 30 F and 92 F. cs.com 1

7 temperature swings are reduced propor onately, and average annual efficiencies are further improved. Balanced loads give us pure geoexchange, where energy rejected during cooling is later retrieved to provide hea ng and there is no net hea ng or cooling of the ground. This represents geothermal at its most sustainable and greatest value. The Problem of Balance The problem of balance is that thermal balance is rarely achieved on its own. A more typical case is illustrated in Figure 3, where the building load is cooling dominated. The thermal load to the ground is even more unbalanced, because the compressor energy is added to the ground during cooling and is deducted from ground heat withdrawal during hea ng. Figure 3. Cooling Dominated Load Profile. Commercial buildings throughout most of North America are cooling dominated. For geothermal systems, the thermal loads to the ground are even more unbalanced because compressor energy is added to the ground during cooling and is deducted from ground heat withdrawal during heating. When annual ground loads are not balanced, heat surpluses or deficits accumulate year a er year. The problem is compounded for mul ple bores due to bore interference. Figure 4 shows the long term ground loop temperature changes in a 6 6 borefield due to ground load imbalances corresponding to various annual cooling to hea ng ra os. The long term loop temperature changes in Figure 4 are computed from the line source model and include all bore to bore interferences. The curves show that even for a building with balanced hea ng and cooling loads (Ra o=1), substan al ground hea ng will s ll occur. To achieve ground thermal balance, the ra o of building s annual cooling to hea ng loads has to be around 0.6 to offset compressor heat. Loop temperatures after the irst year can be calculated by adding the long term temperature changes in Figure 4 to the irst year loop temperatures. It is clear that for virtually any excess annual ground heat, a geothermal bore ield with 150 feet per peak ton and 20 foot bore spacing can result in excessively high loop temperatures after even a few years. Even with annually balanced space loads and a irst year peak EWT of 90, the peak EWT would rise to 101 F after ten years and to 106 F after twenty years. cs.com 2

8 ( F) 150 feet/ton, 20' bore spacing Balanced Space Loads Hea ng: Cooling Ra o (Annual) Figure 4. Long Term Loop Temperature Changes due to Ground Load Imbalance. For a cooling:heating ratio of 0.6, the ground load is balanced, because of the compressor heat. Results are for 150 feet of boring per peak ton of cooling load and a 6 6 bore grid with 20' spacing. Solutions to Imbalance There are two primary ways of reducing long term hea ng effects: 1) add more boring footage, and 2) increase bore spacing. Figure 5 illustrates these effects for the moderate cooling dominated case. Figure 5 shows that doubling the boring footage cuts the long term temperature rise in half. Increasing the bore spacing from 20 feet to 50 feet achieves similar reduc ons, but this results in a six fold increase in the borefield area. If we increase the bore spacing to 50 feet for the moderate cooling dominated case shown in Figure 5, the ground hea ng a er 20 years drops from 21 F with 20 spacing to 9 F. For a first year peak EWT of 90, this would reduce the peak EWT at 20 years from 111 F to 99 F. If instead we increase the boring footage, we not only cut the long term temperature rise, we also reduce the water temperature swings about the undisturbed ground temperature. Doubling the boring footage from 150 feet per ton to 300 feet per ton but keeping the 20 spacing, reduces the ground hea ng a er 20 years from 21 F to 11 F. When we cut the annual swing in half, we get a maximum temperature a er 20 years of 84 F. This example illustrates that for a moderate cooling dominated site, increasing bore spacing helps, but reducing long term borefield hea ng requires substan ally increasing the bore footage. The addi onal footage increases rapidly as cooling dominance increases or the borefield gets more crowded. This can quickly make geothermal uneconomical for cooling dominated applica ons. cs.com 3

9 ( F) Bore feet per ton/bore spacing Cooling: Hea ng = 1.3 Figure 5. Effects of Bore Footage and Spacing on Long Term Heating. All curves are for the moderate cooling dominated case in Figure 4, with an annual cooling:heating ratio of 1.3. Increasing the depths from 150 to 200 or 300 feet per ton reduces long term heating by 25 or 50%. Increasing the spacing from 20 feet to 30 or 50 feet has a similar effect. The GXActive Hybrid Solution The cost of unbalanced loads provides a large incen ve to minimize a building s load imbalance. Energysaving steps not only save money down the road in reduced u lity bills, they can reduce first costs by allowing for a much reduced borefield. This is the obvious first approach to reducing borefield costs. When load imbalance persists, a hybrid geoexchange system may be the best op on. A schema c of a typical hybrid system is shown in Figure 6. A hybrid system combines a closed loop geothermal borefield with some form of supplemental heat rejec on (or supply). Hybrid systems are o en designed with a cooling tower to provide supplemental cooling on hot summer days. While this reduces the cooling load and allows for a propor onate borefield reduc on, this approach generally does not achieve thermal balance. When a borefield is designed for thermal balance, the field size can be greatly reduced while s ll being able to provide for peak cooling loads. This is the GXAc ve solu on. When a borefield is ac vely operated to maintain thermal balance, bores can be spaced as close as 15 feet, with a 40% reduc on in borefield size compared to 20 foot spacing. When the primary goal is to achieve thermal balance, many more op ons for supplemental cooling become available. A dry cooler eliminates the maintenance and water consump on of cooling towers. While they are not efficient for rejec ng peak loads on hot days, dry coolers can be very efficient in the shoulder and winter months, or at night. Water features, whether natural or landscaped, can also cs.com 4

10 Figure 6. Hybrid Schematic. In most hybrid applications, the supplemental fluid cooler comes on when the loop temperatures get too hot. The fluid cooler generally runs when the air is hot, so that evaporative cooling tower is often needed. Running the fluid cooler to actively maintain annual thermal balance provides greater flexibility on the type of cooler and when it is used. provide efficient sources of heat rejec on. Passive warming of sidewalks, drives and parking areas by piping returning water under these surfaces can promote snow and ice melt, providing a beneficial use for heat rejec on. Hybrid systems clearly reduce first costs. They can also reduce energy costs over the life me of the borefield. The smaller hybrid borefield means that temperature swings are higher over the first few years compared to a larger sized borefield. This reduces the system efficiency in the early years compared to a larger geoexchangeonly system. However, in later years, the rising temperature in the geoexchange only system reverses its ini al economic benefits. Figure 7 illustrates this comparison over a 20 year period. The hybrid design not only saves money up front, it can start to save money in opera ons a er only a few years. Figure 7. Energy use over 20 year life of hybrid geoexchange system compared to geoexchange only system. cs.com 5

11 Design, Measurement, and Control Hybrid Geoexchange Primer The keys to maintaining thermally balanced opera on are design, measurement, and control. Design: Once the annual thermal budget is known, the borefield is sized for peak cooling loads and first year EWTs. Supplemental coolers are sized to provide excess heat rejec on over the course of the year. For heat rejec on to the air, the most efficient means of heat rejec on is based on a fixed temperature difference between condenser water temperature and the air temperature. The temperature differen al to achieve energy balance to the ground is ini ally es mated based on hourly coil loads, hourly ground loop temperatures, hourly weather data, and supplemental cooler specifica ons. Measurement: During opera on, heat transfers to and from the ground loop are measured, and a tally of net heat to the ground is maintained. Other informa on such as building loads, heat rejec on by supplemental equipment, and water and air temperatures can be incorporated into algorithms to improve the predic ve power of the thermal balance controls. Control: If thermal imbalance is detected, supplemental fluid cooler se ngs are adjusted. For wet or dry fluid coolers, this can simply be the temperature differen al to control the fluid cooler opera on. Opera onal control of thermal balance maintains efficiency and system robustness. GXAc ve thermal balance strategy allows the system to adjust to changes in building usage or climate. GXActive Bore Optimization Hybrid design allows significant reduc on in field size. However, this means that we are squeezing peak cooling loads through much less bore. This significantly elevates the importance of op mizing the borehole design. The borehole thermal resistance (BTR) is the resistance of the geothermal bore to heat flow. We have no control on the ground itself, but we can design the geothermal bore to reduce its BTR. Figure 8 and Table 1 illustrate the importance of minimizing BTR for hybrid systems. Figure 8. Hourly Loop Temperatures for Different Bore Designs. The calculated hourly loop temperatures are for 150 feet of boring per ton of peak cooling. For a typical bore with a single U bend and thermal grout (K=1.0), temperatures swings with 150 feet of bore per ton are about ±31 F. Non thermal grout adds 13 to that swing, and a highly efficient bore (BTR=0,1) takes 6 off of the swing. Table 1 shows the peak and average EWTs for the bore designs shown in Figure 8. Figure 8 and Table 1 show the large effect of borehole resistance on peak loop temperatures. This is due en rely to temperature difference between the water loop and the bore wall. The forma on temperatures and transfer rates are iden cal in all three cases. cs.com 6

12 Table 1. Effects of Borehole Design and Thermal Resistance on Loop Temperatures Bore Design BTR Max / Avg Cooling EWT Min / Avg Hea ng EWT Single loop, bentonite / 83 F 21 / 40 F Single loop, thermal grout / 76 F 30 / 44 F Twister loop, 5 bore / 72 F 35 / 47 F Taking Geoexchange to the Next Level The promise of geoexchange has always been efficient delivery of hea ng and cooling. However, project owners o en pass on geoexchange because of the high costs of large borefields, needed to address the cooling dominated loads that are typical in commercial buildings throughout most of North America. Hybrid geothermal offers designers and project owners an alterna ve. Ul mately, the three keys to an efficient, robust, and cost effec ve geoexchange system are: 1. Reducing building load imbalance. 2. Ac ve control of supplemental fluid coolers to maintain thermal balance to the ground. 3. Efficient bore designs to reduce peak temperature swings. Designers who follow this path deliver geoexchange systems that lower first costs, are more robust, and consume less energy than geothermal only solu ons. Hybrid design takes geoexchange to the next level. cs.com 7

13 Sterling, Virginia (571) cs.com