Jay Burch, Jim Salasovich, Craig Christensen — National Renewable Energy Laboratory, 1617 Cole Blvd., Golden, CO 80401
Jeff Thornton — Thermal Energy Simulation Specialists, Inc., Madison, WI
This paper presents simulated performance across the U.S. of an unglazed collector system supplying domestic hot water, space heating, and space cooling loads. The collector model is based upon an ISO test for unglazed collectors that explicitly includes effects of wind and sky infrared. To provide realistic loads as location varies, building envelope characteristics vary according to code specifications. Maps of savings for heating and cooling are presented, showing a complex convolution of solar incidence and building load. The U.S. southwest region shows largest savings for both heating and cooling. Average heating efficiency varies from 13 to 28% for collector sizes from 24 to 6 m², respectively. Monetary savings and system cost goals are given for an all-electric and a (natural gas + electric) scenario.
The "triple play" system here is intended to meet domestic hot water (DHW), space heating (HTG), and space cooling (CLG) loads. Solar-driven residential HTG and CLG present significant challenges: HTG is needed in winter when solar radiation is lowest; low winter sun angles reduce incidence for roof-flat collectors or require rack-mounting; HTG is a "part-time" load that shrinks as envelopes improve. Thermally-driven cooling is costly and operates at higher temperatures where flat plates are inefficient. In the U.S., glazed solar systems are almost never used for HTG or CLG loads.
Interest exists in unglazed systems because: (i) lower cost/benefit and improved aesthetics vs. glazed systems; (ii) unglazed PV-thermal hybrid collectors may work well together and are more aesthetic than separate systems. Collector cost could approach polymer pool collectors, at under $1/ft² FOB, versus ~$15/ft² for glazed. Unglazed collectors could be molded to "look like" an ordinary roof, similar in spirit to the "invisible collector" (3). In new construction with roof-integrated systems, net cost would be aided by credits for eliminating regular roofing. Along with these advantages comes diminished and more uncertain heating performance; good performance data are key for economic and market assessments. This paper presents first results of simulating an unglazed system meeting code-minimum-house loads across the U.S., with collector areas from 6 to 24 m², and details monetary savings and cost goals.
The solar system includes an unglazed collector array, hot and cold storage tanks, and controls with liquid-to-air heat exchangers. The model is implemented in TRNSYS (4); the unglazed collector model is described in (5,6), with performance expressed in terms of radiation, wind, and temperature. Collector coefficients are taken from (6) for 'FA357.' Key parameters: floor area 186 m², window 33 m²; infiltration 0.57 ACH; DHW setpoint 55°C, draw 242 L/day; solar HTG/CLG setpoints 21.1/23.9°C; collector slope 30°; panel area 2.92 m²; flow rate 248 kg/hr-m²; tank storage 80 kg/m²-collector; multi-stage thermostat stages solar first, then auxiliary. The house is a one-story, slab-on-grade, single-zone model; key house parameters vary with site heating degree days (HDD) per code (7). Window properties (U, SHGC) vary with HDD (Table 2). HTG/CLG data below are energy to zone; divide by equipment efficiency for auxiliary fuel.
Simulations at all TMY2 sites (10) for collector areas 6, 12, and 24 m². Unit-area savings q_sav in GJ/m²-yr; geographical variation is nearly identical for the three areas. Table 3 (summary): Average q_sav for DHW 0.98–0.34, HTG 0.56–0.40, CLG 0.36–0.20 (6 to 24 m²); maxima higher. Maps (Figs. 2, 3) show heating and cooling savings; contours are general trends (238 sites, sparse for fine detail). Savings highest in U.S. southwest, peaking near CO–NM border and extending through CA central valley; cooling savings about 1/4 of heating. Heating efficiency η_HT = (total savings)/(total incidence on collector). For HDD > 3000 °F-days, η_HT averages 25%, 18%, and 13% for 6, 12, and 24 m²; at small size ~60% of a small glazed SDHW system (11). η_HT decreases by a factor ~2 as area increases by 4; Q_sav,HT increases by a factor of 2.
Cost savings depend on whether natural gas is used (gas ~1/3 cost of electric heat). Assumptions: electricity 10 ¢/kWh, natural gas 80 ¢/therm, gas efficiency 70%, chiller COP 3. Fig. 7 shows unit-area monetary savings; highest in the southwest, second peak in SC and westward. Cost goals: Simple payback SP = (system cost)/(annual savings); at SP = 10 years, cost goal = 10 × annual savings. With natural gas, cost goals $100/m² to $230/m²; with electricity only, savings ~2.3× higher, goals ~$230/m² to ~$530/m². Compare to today's solar water heating ~$600/m²; goals are difficult without low-cost collectors and storage. Roof-replacement credits of order ~$50/m² can help for new construction and roof-integrated systems.
DHW, HTG, and CLG savings were computed at all TMY2 locations for 6, 12, and 24 m². Savings are upper limits (parasitics ignored). Heat savings averaged about 1 GJ/m², cooling about 1/4 as much. Savings highest in U.S. southwest. Annual savings ranged ~$10/m² to ~$23/m² (~$23/m² to ~$53/m²) for (natural gas + electric) vs. all-electric. Cost goals set at 10× annual savings; cost goals for natural gas appear unattainable with today's system costs.
The authors acknowledge the support of the U.S. DOE Solar Program. The Solar Thermal program is managed by Tex Wilkins and Glen Strahs. Steve Baer, CEO of Zomeworks, Albuquerque, NM, is acknowledged for pointing out the advantages of unglazed systems and motivating our work.
Source: NREL / ASES conference paper. Text from PDF.
PDF: 2003-01-01-geographical-variation-unglazed.pdf